Thread: Microbiology Case Studies

Case 1 sounds mismanaged, she should have been given empirical treatment, immediately on suspicion, but it was left until after CSF had been collected. Guess the author wrote for an audience with different guidelines than the UK. Here it'd been benzylpenicillin from first medical staff to see her (even if seen by GP, she should have been jabbed while waiting for ambulance). Also, I disagree with kats about the: H Influenza is immunized against at the age of 2 months in both US and UK and the information given includes "Her childhood immunizations were current."

Lange Microbiology and Immunology Review > Part X: Clinical Cases



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Clinical Cases: Introduction

These brief clinical case vignettes are typical presentations of common infectious diseases. Learning the most likely causative organisms of these classic cases will help you answer the USMLE questions and improve your diagnostic skills. These cases are presented in random order similar to the way they are on the USMLE. The important features of the case are written in boldface.

Case 1

A 22-year-old woman has a severe sore throat. Findings on physical exam include an inflamed throat, swollen cervical lymph nodes, and an enlarged spleen. Her heterophile agglutinin test (Monospot test) is positive.

DIAGNOSIS: Infectious mononucleosis caused by Epstein-Barr virus. Other viruses and bacteria, especially Streptococcus pyogenes, can cause pharyngitis and cervical lymphadenopathy, but an enlarged spleen and a positive Monospot test make infectious mononucleosis the most likely diagnosis. See Epstein-Barr Virus (EBV) for additional information.

Case 2

A 5-year-old boy with diabetic ketoacidosis has ptosis of his right eyelid, periorbital swelling, and a black, necrotic skin lesion under his eye. Biopsy of the skin lesion shows nonseptate hyphae with wide-angle branching.

DIAGNOSIS: Mucormycosis caused by Mucor or Rhizopus species. Diabetic ketoacidosis and renal acidosis predispose to mucormycosis. Fungal spores are inhaled into the sinuses, resulting in lesions on the face. See Mucor & Rhizopus for additional information.

Case 3

A 40-year-old man complains of watery, foul-smelling diarrhea and flatulence for the past 2 weeks. He drank untreated water on a camping trip about a month ago. See pear-shaped flagellated trophozoites in stool.

DIAGNOSIS: Giardiasis caused by Giardia lamblia. Of the protozoa that are common causes of diarrhea, Giardia and Cryptosporidium cause watery diarrhea, whereas Entamoeba causes bloody diarrhea. See Giardia for additional information on Giardia, Cryptosporidium for additional information on Cryptosporidium, and Entamoeba for additional information on Entamoeba.

Case 4

A 35-year-old man who is HIV-antibody-positive has had a persistent headache and a low-grade fever (temperature, 100°F) for the past 2 weeks. See budding yeasts with a wide capsule in India ink preparation of spinal fluid.

DIAGNOSIS: Meningitis caused by Cryptococcus neoformans. The latex agglutination test, which detects the capsular polysaccharide antigen of Cryptococcus in the spinal fluid, is a more sensitive and specific test than is the test with India ink. See Laboratory Diagnosis for additional information. If acid-fast rods are seen in spinal fluid, think Mycobacterium tuberculosis. See Mycobacterium tuberculosis for additional information.

Case 5

A 12-year-old boy has a painful arm that he thought he had injured while pitching in a Little League baseball game. The pain has gotten worse over a 2-week period and he now has a temperature of 100°F. X-ray of the humerus reveals raised periosteum. Aspirate of lesion reveals gram-positive cocci in clusters.

DIAGNOSIS: Osteomyelitis caused by Staphylococcus aureus. This organism is the most common cause of osteomyelitis in children. Osteomyelitis in prosthetic joints is often caused by Staphylococcus epidermidis. See Staphylococcus for additional information on staphylococci.

Case 6

A 50-year-old woman receiving chemotherapy via a subclavian catheter for acute leukemia has the sudden onset of blindness in her right eye. Her total WBC = 120/L. Blood cultures grew budding yeasts that formed germ tubes.

DIAGNOSIS: Endophthalmitis (infection inside the eye) caused by Candida albicans. A catheter-related infection gave rise to an embolus containing the organism, which traveled through the bloodstream to reach the eye. C. albicans is a member of the normal flora of the skin and enters through a break in the skin at the catheter site. See Candida for additional information.

If the blood culture grew colonies of gram-positive cocci in clusters that were coagulase-negative, think Staphylococcus epidermidis, another member of the skin flora that is also a common cause of catheter-associated infections. See Staphylococcus for additional information.

Case 7

A 60-year-old man has had a nonproductive cough and fever (temperature, 101°F) for 1 week. He received a kidney transplant 6 weeks ago and has had one episode of rejection that required increased prednisone. There was no response to erythromycin, indicating that Legionella and Mycoplasma are unlikely causes. See owl's-eye inclusion bodies within the nucleus of infected cells in bronchoalveolar lavage fluid.

DIAGNOSIS: Cytomegalovirus pneumonia. These intranuclear inclusions are typical findings in CMV infections. Immunosuppression predisposes to disseminated CMV infections. See Cytomegalovirus (CMV) for additional information.

Case 8

A 45-year-old woman complains that her right arm has become increasingly weak during the past few days. This morning, she had a generalized seizure. She recently finished a course of cancer chemotherapy. MRI of the brain reveals a lesion resembling an abscess. Brain biopsy shows gram-positive rods in long filaments. Organism is weakly acid-fast.

DIAGNOSIS: Brain abscess caused by Nocardia asteroides. N. asteroides initially infects the lung, where it may or may not cause symptoms in immunocompetent people. Dissemination to the brain is common in immunocompromised patients. See Nocardia asteroides for additional information.

Case 9

A 20-year-old man has a severe headache and vomiting that began yesterday. He is now confused. On exam, his temperature is 39°C and his neck is stiff. Spinal fluid reveals no bacteria on Gram stain, 25 lymphs, normal protein, and normal glucose. Culture of the spinal fluid on blood agar shows no bacterial colonies.

DIAGNOSIS:Viral meningitis, which is most often caused by coxsackievirus. Can isolate the virus from spinal fluid. See Coxsackieviruses for additional information.

Case 10

A 60-year-old man with a history of tuberculosis now has a cough productive of bloody sputum. Chest x-ray reveals a round opaque mass within a cavity in his left upper lobe. Culture of the sputum grew an organism with septate hyphae that had straight, parallel walls. The hyphae exhibited low-angle branching.

DIAGNOSIS: "Fungus ball" caused by Aspergillus fumigatus. Fungal spores are inhaled into the lung, where they grow within a preexisting cavity caused by infection with Mycobacterium tuberculosis. See Aspergillus for additional information.

Case 11

A 3-month-old girl has watery, nonbloody diarrhea. Stool culture reveals only normal enteric flora.

DIAGNOSIS: Think rotavirus, the most common cause of diarrhea in infants. The ELISA test for rotavirus antigen in the stool is positive, which confirms the diagnosis. See Rotavirus for additional information.

Case 12

A 30-year-old woman has a painless ulcer on her tongue. She is HIV-antibody-positive and has a CD4 count of 25. Her serum is nonreactive in the VDRL test. Biopsy of the lesion revealed yeasts within macrophages.

DIAGNOSIS: Disseminated histoplasmosis caused by Histoplasma capsulatum. Patients with a low CD4 count have severely reduced cell-mediated immunity, which predisposes to disseminated disease caused by this dimorphic fungus. A negative VDRL test indicates the ulcer was not caused by Treponema pallidum. See Histoplasma for additional information on Histoplasma.

Case 13

A 20-year-old man has a swollen, red, hot, tender ankle, accompanied by a temperature of 100°F for the past 2 days. There is no history of trauma. See gram-negative diplococci in joint fluid aspirate. Organism is oxidase-positive.

DIAGNOSIS: Arthritis caused by Neisseria gonorrhoeae, the most common cause of infectious arthritis in sexually active adults. Sugar fermentation tests were used to identify the organism as N. gonorrhoeae. See Neisseria gonorrhoeae for additional information.

Case 14

A 40-year-old woman has blurred vision and slurred speech. She is afebrile. She is famous in her neighborhood for her home-canned vegetables and fruits.

DIAGNOSIS: Botulism caused by Clostridium botulinum. Botulinum toxin causes a descending paralysis that starts with the cranial nerves, typically appearing initially as diplopia. The toxin is a protease that cleaves the proteins involved in the release of acetylcholine at the neuromuscular junction. Treat with antiserum immediately. Confirm diagnosis with mouse protection test using a sample of food suspected of containing the toxin. See Clostridium botulinum for additional information.

Wound botulism occurs in heroin users (e.g., users of black tar heroin), especially in those who "skin pop." Bacterial spores in the heroin germinate in the anaerobic conditions in necrotic skin tissue.

Case 15

A neonate was born with a small head (microcephaly), jaundice, and hepatosplenomegaly. Urine contained multinucleated giant cells with intranuclear inclusions.

DIAGNOSIS: Cytomegalovirus infection acquired in utero. Cytomegalovirus is the leading cause of congenital abnormalities. For fetal infection to occur, the mother must be infected for the first time during pregnancy. She therefore would have no preexisting antibodies to neutralize the virus prior to its infecting the placenta and the fetus. See Cytomegalovirus (CMV) for additional information.

Case 16

A 14-year-old girl has a rapidly spreading, painful, erythematous rash on her leg. The rash is warm and tender, and her temperature is 38°C. Gram-positive cocci in chains were seen in an aspirate from the lesion. Culture of the aspirate on blood agar grew colonies surrounded by clear (beta) hemolysis. Growth of the organism was inhibited by bacitracin.

DIAGNOSIS: Cellulitis caused by Streptococcus pyogenes. The rapid spread of cellulitis caused by S. pyogenes is due to hyaluronidase (spreading factor) that degrades hyaluronic acid in subcutaneous tissue. Acute glomerulonephritis (AGN) can follow skin infections caused by S. pyogenes. AGN is an immunologic disease caused by antigen–antibody complexes. See Diseases for additional information.

Case 17

A 4-year-old boy wakes up at night because his anal area is itching. See worm eggs in "Scotch tape" preparation.

DIAGNOSIS: Pinworm infection (enterobiasis) caused by Enterobius vermicularis. Pinworm infection is the most common helminth disease in the United States. See Enterobius for additional information.

Case 18

A 25-year-old woman has a painful, inflamed swollen hand. She was bitten by a cat about 8 hours ago. See small gram-negative rods in the exudate from lesion.

DIAGNOSIS: Cellulitis caused by Pasteurella multocida. Organism is normal flora in cat's mouth. See Pasteurella for additional information.

Case 19

A 7-year-old girl has bloody diarrhea and fever (temperature, 38°C), but no nausea or vomiting. Only lactose-fermenting colonies are seen on EMB agar.

DIAGNOSIS: Think Campylobacter jejuni or enterohemorrhagic strains of Escherichia coli (E. coli O157:H7). If Campylobacter is the cause, see colonies on Campylobacter agar containing curved gram-negative rods, and the colonies on EMB agar are likely to be nonpathogenic E. coli. If E. coli O157:H7 is the cause, the organism in the lactose-fermenting colonies on EMB agar is unable to ferment sorbitol. The absence of non-lactose-fermenting colonies indicates that Shigella and Salmonella are not the cause. See Campylobacter for additional information on Campylobacter and Pathogenesis for additional information on E. coli O157:H7.

Case 20

A 15-year-old girl has had a nonproductive cough and temperature of 100°F for the past 5 days. The symptoms came on gradually. Lung exam shows few scattered rales. Chest x-ray shows patchy infiltrate in left lower lobe but no consolidation. Cold agglutinin test is positive.

DIAGNOSIS: Atypical pneumonia caused by Mycoplasma pneumoniae. This organism is the most common cause of atypical pneumonia in teenagers and young adults. In the cold agglutinin test, antibodies in the patient's serum agglutinate human red blood cells in the cold (4°C). These antibodies do not react with Mycoplasma. See Mycoplasma pneumoniae for additional information.

Case 21

A 45-year-old man sustained a skull fracture in an automobile accident. The following day, he noted clear fluid dripping from his nose, but he did not notify the hospital personnel. The following day he spiked a fever to 39°C and complained of a severe headache. Nuchal rigidity was found on physical exam. Spinal fluid analysis revealed 5200 WBC/L, 90% of which were neutrophils. Gram stain showed gram-positive diplococci.

DIAGNOSIS: Meningitis caused by Streptococcus pneumoniae. Patients with a fracture of the cribriform plate who leak spinal fluid into the nose are predisposed to meningitis by this organism. Pneumococci can colonize the nasal mucosa and enter the subarachnoid space through the fractured cribriform plate. See Streptococcus pneumoniae for additional information.

Case 22

A 7-year-old girl was well until about 3 weeks ago, when she began complaining of being "tired all the time." On exam, her temperature is 38°C and there is tenderness below the right knee. Hemoglobin: 10.2; WBC: 9600 with increased neutrophils. A sickle cell prep shows a moderate sickling tendency. Gram-negative rods grew in the blood culture.

DIAGNOSIS: Osteomyelitis caused by Salmonella species. Sickle cell anemia predisposes to osteomyelitis caused bySalmonellaspecies. The abnormally shaped sickle cells are trapped in the small capillaries of the bone and cause micro-infarcts. These micro-infarcts enhance the likelihood of infection by Salmonella. See Salmonella for additional information.

Case 23

A 3-month-old boy has a persistent cough and severe wheezing for the past 2 days. On physical exam, his temperature is 39°C and coarse rhonchi are heard bilaterally. Chest x-ray shows interstitial infiltrates bilaterally. Diagnosis was made by ELISA that detected viral antigen in nasal washings.

DIAGNOSIS: Think pneumonia caused by respiratory syncytial virus, the most common cause of pneumonia and bronchiolitis in infants. RSV causes giant cells (syncytia) that can be seen in respiratory secretions and in cell culture. See Respiratory Syncytial Virus for additional information.

Case 24


A 34-year-old man was in his usual state of health until last night, when he felt feverish, had a shaking chill, and became short of breath at rest. T 39°C, BP 110/60, P 104, R 18. Scattered rales were heard in both bases. A new murmur consistent with tricuspid insufficiency was heard. Needle tracks were seen on both forearms. Gram-positive cocci in clusters grew in blood culture.

DIAGNOSIS: Acute endocarditis caused by Staphylococcus aureus. This organism is the most common cause of acute endocarditis in intravenous drug users. The valves on the right side of the heart are often involved. See Staphylococcus aureus: Kawasaki Syndrome for additional information.

Case 25

A 2-week-old infant was well on discharge from the hospital 10 days ago and remained so until last night, when he appeared drowsy and flushed. His skin felt hot to the touch. On physical exam, the infant was very difficult to arouse, but there were no other positive findings. His temperature was 40°C. Blood culture grew gram-positive cocci in chains. A narrow zone of clear (beta) hemolysis was seen around the colonies. Hippurate hydrolysis test was positive.

DIAGNOSIS: Neonatal sepsis caused by Streptococcus agalactiae (group B streptococci). Group B streptococci are the most common cause of neonatal sepsis. Think Escherichia coli if gram-negative rods are seen or Listeria monocytogenes if gram-positive rods are seen. See Important Properties for additional information on group B streptococci, Escherichia for additional information on Escherichia coli, and Listeria monocytogenes for additional information on Listeria monocytogenes.

Case 26

A 70-year-old woman had a hip replacement because of severe degenerative joint disease. She did well until a year later, when a fall resulted in a fracture of the femur and the prosthesis had to be replaced. Three weeks later, bloody fluid began draining from the wound site. The patient was afebrile and the physical examination was otherwise unremarkable. Two days later, because of increasing drainage, the wound was debrided and pus was obtained. Gram stain of the pus was negative, but an acid-fast stain revealed red rods.

DIAGNOSIS: Prosthetic joint infection caused by Mycobacterium fortuitum-chelonei complex. Think Staphylococcus epidermidis if gram-positive cocci in clusters are seen. See Group IV (Rapidly Growing Mycobacteria) for additional information on Mycobacterium fortuitum-chelonei complex and Staphylococcus epidermidis & Staphylococcus saprophyticus for additional information on Staphylococcus epidermidis.

Case 27

An 80-year-old man complains of a painful rash on his left forehead. The rash is vesicular and only on that side. He is being treated with chemotherapy for leukemia. Smear of material from the base of the vesicle reveals multinucleated giant cells with intranuclear inclusions.

DIAGNOSIS: Herpes zoster (shingles) caused by varicella-zoster virus. The rash of zoster follows the dermatome of the neuron that was latently infected. Herpes simplex type 1 virus can cause a similar picture. Can distinguish these viruses using fluorescent antibody assay. See Varicella-Zoster Virus (VZV) for additional information.

Case 28

A 55-year-old woman has an inflamed ulcer on her right hand and several tender nodules on the inner aspect of her right arm. She is an avid gardener and especially enjoys pruning her roses. Biopsy of the lesion reveals budding yeasts.

DIAGNOSIS: Sporotrichosis caused by Sporothrix schenckii. The organism is a mold in the soil and a yeast in the body, i.e., it is dimorphic. Infection occurs when spores produced by the mold form are introduced into the skin by a penetrating injury. See Sporotrichosis for additional information.

Case 29

A 15-year-old boy sustained a broken tooth in a fist fight several weeks ago. He now has an inflamed area on the skin over the broken tooth, in the center of which is a draining sinus tract. Gram stain of the drainage fluid reveals filamentous gram-positive rods.

DIAGNOSIS: Actinomycosis caused by Actinomyces israelii. See "sulfur granules" in the sinus tract. These granules are particles composed of interwoven filaments of bacteria. See Actinomyces israelii for additional information.

Case 30

A 24-year-old woman experienced the sudden onset of high fever, myalgias, vomiting, and diarrhea. Her vital signs were T 40°C, BP 70/30, P 140, R 30. A sunburn-like rash appeared over most of her body. Blood cultures and stool cultures are negative. She is recovering from a surgical procedure on her maxillary sinus and the bleeding was being staunched with nasal tampons. Gram-positive cocci in clusters were seen in blood adherent to the nasal tampon.

DIAGNOSIS: Toxic shock syndrome caused by Staphylococcus aureus. Toxic shock syndrome toxin is a superantigen that stimulates the release of large amounts of cytokines from many helper T cells. See Pathogenesis for additional information.

Case 31

An 8-year-old girl has a pruritic rash on her chest. Lesions are round or oval with an inflamed border and central clearing. The lesions contain both papules and vesicles. See hyphae in KOH prep of scrapings from the lesion.

DIAGNOSIS: Tinea corporis (ringworm) caused by one of the dermatophytes, especially species of Microsporum, Trichophyton, or Epidermophyton. Dermatophytes utilize keratin as a nutrient source, so lesions are limited to the skin. See Dermatophytoses for additional information.

Case 32

A 25-year-old woman has a papular rash on her trunk, arms, and palms. She says the rash does not itch. Vaginal exam reveals two flat, moist, slightly raised lesions on the labia. Material from a labial lesion examined in a darkfield microscope revealed spirochetes.

DIAGNOSIS: Secondary syphilis caused by Treponema pallidum. The rash on the palms coupled with the vaginal lesions (condylomata lata) are compatible with secondary syphilis. Serologic tests, such as the nonspecific test (VDRL) and the specific test (FTA-ABS), were positive. See Treponema pallidum for additional information.

Case 33

A 5-year-old girl complains of an earache for the past 2 days. On examination, she has a temperature of 39°C, the right external canal contained dried blood, the drum was perforated, and a small amount of purulent fluid was seen. Gram stain of the pus revealed gram-positive diplococci. Colonies formed green (alpha) hemolysis on blood agar. Growth was inhibited by optochin.

[COLOR="blue"]DIAGNOSIS:[/COLOR] Otitis media caused by Streptococcus pneumoniae. Think Haemophilus influenzae if small gram-negative rods are seen. These organisms colonize the oropharynx and enter the middle ear via the eustachian tube. See Streptococcus pneumoniae for additional information on Streptococcus pneumoniae and Haemophilus for additional information on Haemophilus influenzae.

Case 34

A 25-year-old woman was well until the sudden onset of high fever (temperature, 40°C) accompanied by several purple skin lesions (ecchymoses, purpura). The lesions are scattered over the body, are irregularly shaped, and are not raised. Her blood pressure is 60/10 and her pulse rate is 140. Blood culture grew gram-negative diplococci.

DIAGNOSIS: Meningococcemia caused by Neisseria meningitidis. The endotoxin (lipopolysaccharide, or LPS) of the organism triggers release of interleukin-1, tumor necrosis factor, and nitric oxide from macrophages. These cause the high fever and low blood pressure. The purpuric lesions are a manifestation of disseminated intravascular coagulation (DIC). Endotoxin activates the coagulation cascade, causing DIC. Lipid A is the toxic part of LPS. See Neisseria meningitidis for additional information.

Case 35

A 40-year-old woman was well until 2 days ago, when she experienced the sudden onset of fever, shaking chills, and profuse sweating. Today, she also complains of headache and abdominal pain but no nausea, vomiting, or diarrhea. She does not have a stiff neck, rash, or altered mental status. Travel history reveals she returned from an extended trip to several countries in Central Africa 1 week ago. Blood smear reveals ring-shaped trophozoites within red blood cells.

DIAGNOSIS: Malaria caused by Plasmodium species. If banana-shaped gametocytes seen in the blood smear, think Plasmodium falciparum. P. falciparum is the species that causes the life-threatening complications of malaria, such as cerebral malaria. The fever and chills experienced by the patient coincide with the release of merozoites from infected red blood cells and occur in either a tertian or quartan pattern. See Plasmodium for additional information.

Case 36

A 35-year-old man is seen in the emergency room complaining of severe headache and vomiting that began last night. His temperature is 40°C. While in the ER, he is increasingly combative and has a grand-mal seizure. He is "foaming at the mouth" and cannot drink any liquids. Analysis of his spinal fluid reveals no abnormality, and no organisms are seen in the Gram stain. Two days later, despite supportive measures, he died. Pathologic examination of the brain revealed eosinophilic inclusion bodies in the cytoplasm of neurons.

DIAGNOSIS: Rabies (an encephalitis) caused by rabies virus. The inclusions are Negri bodies. Diagnosis can be confirmed by using fluorescent-antibody assays. The patient was a farm worker who was bitten by a bat about a month prior to the onset of symptoms. Note the long incubation period, which can be as long as 6 months. People bitten by a bat (or any wild animal) should receive rabies immunization consisting of the inactivated vaccine plus rabies immune globulins (passive-active immunization). See Rabies Virus for additional information.

Case 37

A 70-year-old man was admitted to the hospital after suffering extensive third-degree burns. Three days later, he spiked a fever, and there was pus on the dressing that had a blue-green color. Gram stain of the pus revealed gram-negative rods.

DIAGNOSIS: Wound (burn) infection caused by Pseudomonas aeruginosa. The blue-green color is caused by pyocyanin, a pigment produced by the organism. See Pseudomonas for additional information.

Case 38

A 65-year-old woman reports that she has had several episodes of confusion and memory loss during the past few weeks. On examination, she is afebrile but has a staggering gait and myoclonus can be elicited. Over the next several months, her condition deteriorates and death ensues. On autopsy, microscopic examination of the brain reveals many vacuoles but no viral inclusion bodies.

DIAGNOSIS: Creutzfeldt-Jakob disease caused by prions. CJD is a spongiform encephalopathy. The vacuoles give the brain a sponge-like appearance. See Creutzfeldt-Jakob Disease for additional information.

Case 39

A 20-year-old man complains of several episodes of blood in his urine. He has no dysuria or urethral discharge. He is not sexually active. He is a college student but was born and raised in Egypt. Physical exam reveals no penile lesions. Urinalysis shows many red cells, no white cells, and several large eggs with terminal spines.

DIAGNOSIS: Schistosomiasis caused by Schistosoma haematobium. Schistosome eggs in venules of the bladder damage the bladder epithelium and cause bleeding. The eggs are excreted in the urine. See Schistosoma for additional information.

Case 40

A 35-year-old man complains of night sweats, chills, and fatigue at varying intervals during the past 2 months. These episodes began while he was traveling in Latin America. When questioned, he says that cheeses, especially the unpasteurized varieties, are some of his favorite foods. On examination, his temperature is 39°C, and his liver and spleen are palpable. His hematocrit is 30% and his white blood count is 5000. Blood culture grew small gram-negative rods.

DIAGNOSIS: Brucellosis caused by Brucella species. Domestic animals such as cows and goats are the main reservoir for Brucella, and it is often transmitted in unpasteurized dairy products. This patient could also have typhoid fever caused by Salmonella typhi, but S. typhi is only a human pathogen, i.e., there is no animal reservoir. See Brucella for additional information on Brucella species and Salmonella for additional information on Salmonella typhi.

Case 41

A 6-year-old girl has a rash on her face that appeared yesterday. The rash is erythematous and located over the malar eminences bilaterally. The rash is macular; there are no papules, vesicles, or pustules. A few days prior to the appearance of the rash, she had a runny nose and anorexia.

DIAGNOSIS: Slapped cheek syndrome caused by parvovirus B19. This virus also causes aplastic anemia because it preferentially infects and kills erythroblasts. It also infects the fetus, causing hydrops fetalis, and causes an immune complex–mediated arthritis, especially in adult women. See Parvoviruses for additional information.

Case 42

A 20-year-old man fell off his motorcycle and suffered a compound fracture of the femur. The fracture was surgically reduced and the wound debrided. Forty-eight hours later, he spiked a fever (temperature, 40°C) and the wound area became necrotic. Crepitus was felt, and a foul-smelling odor was perceived originating from the wound. Marked anemia and a white blood count of 22,800 were found. Gram stain of the exudate showed large gram-positive rods. Colonies grew on blood agar incubated anaerobically but not aerobically.

DIAGNOSIS: Gas gangrene (myonecrosis) caused by Clostridium perfringens. The main virulence factor produced by this organism is an exotoxin that is a lecithinase. It causes necrosis of tissue and lysis of red blood cells (causing hemolytic anemia). The spores of the organism are in the soil and enter at the wound site. A foul-smelling exudate is characteristic of infections caused by anaerobic bacteria. See Disease: Gas Gangrene for additional information.

Case 43

A 30-year-old woman complains of a burning feeling in her mouth and pain on swallowing. Sexual history reveals she is a commercial sex worker and has had unprotected vaginal, oral, and anal intercourse with multiple partners. On examination, whitish lesions are seen on the tongue, palate, and pharynx. No vesicles are seen. The test for HIV antibody is positive and her CD4 count is 65. Gram stain of material from the lesions reveals budding yeasts and pseudohyphae.

DIAGNOSIS: Thrush caused by Candida albicans. This organism forms pseudohyphae when it invades tissue. The absence of vesicles indicates that her symptoms are not caused by herpes simplex virus type 2. See Candida for additional information.

Case 44

You're a physician at a refugee camp in sub-Saharan Africa, when an outbreak of diarrhea occurs. Massive amounts of watery stool, without blood, are produced by the patients. Curved gram-negative rods are seen in a Gram stain of the stool.

DIAGNOSIS: Cholera caused by Vibrio cholerae. There are three genera of curved gram-negative rods: Vibrio, Campylobacter, and Helicobacter.Vibrio cholerae causes watery, nonbloody diarrhea, whereas Campylobacter jejuni typically causes bloody diarrhea. Helicobacter pylori causes gastritis and peptic ulcer, not diarrhea.

Enterotoxigenic Escherichia coli causes watery diarrhea by producing an exotoxin that has the same mode of action as does the exotoxin produced by V. cholerae. However, E. coli is a straight gram-negative rod, not a curved one. If an outbreak of bloody diarrhea had occurred in the refugee camp, then Shigella dysenteriae would be the most likely cause. See the following pages for additional information: Vibrio ; Campylobacter ; Helicobacter ; Escherichia ; and Shigella .

Case 45

A 40-year-old man with low-grade fever and night sweats for the past 4 weeks now has increasing fatigue and shortness of breath. He says he has difficulty climbing the one flight of stairs to his apartment. Pertinent past history includes rheumatic fever when he was 15 years old and the extraction of two wisdom teeth about 3 weeks before his symptoms began. No chemoprophylaxis was given at the time of the extractions. There is no history of intravenous drug use. His temperature is 38.5°C, and a loud holosystolic murmur can be heard over the precordium. His spleen is palpable. He is anemic and his white blood count is 13,500. Blood cultures grow gram-positive cocci in chains that produce green (alpha) hemolysis on blood agar. Growth is not inhibited by optochin.

DIAGNOSIS: Subacute bacterial endocarditis caused by one of the viridans group streptococci, such as Streptococcus sanguis. The laboratory findings are also compatible with Enterococcus faecalis, but the history of dental surgery makes the viridans group streptococci more likely to be the cause. Endocarditis caused by E. faecalis is associated with GI or GU tract surgery. See Clinical Findings for additional information on both viridans group streptococci and E. faecalis.

Case 46

A 60-year-old woman is asymptomatic but has a lung nodule seen on chest x-ray. Pertinent past history includes her cigarette smoking (2 packs per day for 40 years) and her occupation as an archaeologist, digging primarily in Arizona and New Mexico. Because of concern that the nodule may be malignant, it was surgically removed. Pathologic examination revealed large (25 ) round structures with thick walls and many round spores inside. No malignant cells were seen.

DIAGNOSIS: Coccidioidomycosis caused by Coccidiodes immitis. These structures are spherules, which are pathognomonic for this disease. The mold form of the organism is found in the soil of the Southwestern United States, and the organism is acquired by inhalation of arthrospores produced by the mold. The inhaled arthrospores form spherules in the lung. C. immitis is dimorphic and forms spherules at 37°C. See Coccidioides for additional information.

Case 47

A 20-year-old woman in her 30th week of pregnancy had an ultrasound examination that revealed a growth-retarded fetus with a large head (indicating hydrocephalus) and calcifications within the brain. Umbilical blood was cultured and crescent-shaped trophozoites were grown.

DIAGNOSIS: Toxoplasmosis caused by Toxoplasma gondii. Detection of IgM antibody in the Sabin-Feldman dye test can also be used to make a diagnosis. The main reservoir is domestic cats. Domestic farm animals, such as cattle, acquire the organism by accidentally eating cat feces. Pregnant women should not be exposed to cat litter or eat undercooked meat. See Toxoplasma for additional information.

Case 48

A 10-day-old neonate has several vesicles on the scalp and around the eyes. The child is otherwise well, afebrile and feeding normally. A Giemsa-stained smear of material from the base of a vesicle revealed multinucleated giant cells with intranuclear inclusions.

DIAGNOSIS: Neonatal infection caused by herpes simplex virus type 2. Infection is acquired during passage through the birth canal. Life-threatening encephalitis and disseminated infection of the neonate also occur. See Herpes Simplex Virus (HSV) for additional information.

Case 49

A 40-year-old woman has just had a grand-mal seizure. There is a history of headaches for the past week and one episode of vertigo but no previous seizures. She is afebrile. She is a native of Honduras but has lived in the United States for the past 5 years. MRI reveals a mass in the parietal lobe. Surgical removal of the mass reveals a larva within a cyst-like sac.

DIAGNOSIS: Cysticercosis caused by the larva of Taenia solium. Infection is acquired by ingesting the tapeworm eggs, not by ingesting undercooked pork. This clinical picture can also be caused by a brain abscess, a granuloma such as a tuberculoma, or a brain tumor. See Taenia solium for additional information.

Case 50

A 1-week-old neonate has a yellowish exudate in the corners of both eyes. The child is otherwise well, afebrile and feeding normally. Gram stain of the exudate reveals no gram-negative diplococci. A Giemsa-stained smear of the exudate reveals a large cytoplasmic inclusion.

DIAGNOSIS: Conjunctivitis caused by Chlamydia trachomatis. Confirm the diagnosis with direct fluorescent antibody test. Infection is acquired during passage through the birth canal. The inclusion contains large numbers of the intracellular replicating forms called reticulate bodies. See Chlamydiae for additional information.

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