Test yourself: Infectious diseases

The following cases and commentary, which address infectious diseases, are excerpted from ACP's Medical Knowledge Self-Assessment Program (MKSAP14).

Case 1: Seafood restaurant worker

A 63-year-old man is brought to the emergency department because of a 1-day history of rapidly progressing pain, swelling and erythema of his left hand associated with fever, chills and delirium. The patient works in a restaurant on the Gulf Coast of Florida preparing seafood. One day ago, he noted redness and swelling of his left thumb in an area that had been abraded by an oyster shell. The area of redness quickly spread proximally and has now progressed beyond the elbow. Hemorrhagic bullae formed, the skin on the hand and arm began to darken, and the patient became delirious. Medical history is unremarkable.

On physical examination, the patient is awake but is disoriented and is writhing in pain. Temperature is 39.8°C (103.6°F), pulse rate is 122/min, respiration rate is 24/min, and blood pressure is 88/40 mm Hg. The left arm and hand show the changes described above. In addition, necrosis of skin up to the upper arm has developed. The remainder of the examination is unremarkable. Laboratory studies on admission were as follows: hemoglobin level, 7.2 g/dL (72 g/L); hematocrit, 21%; leukocyte count, 2800/µL (2.8 × 109/L) with 88% immature neutrophils, 11% mature neutrophils, and 1% lymphocytes; platelet count, 45,000/µL (45 × 109/L); serum sodium, 122 mEq/L (122 mmol/L); serum potassium, 3.2 mEq/L (3.2 mmol/L); and serum chloride, 92 mEq/L (92 mmol/L).

Which of the following pathogens is most likely causing this patient's current findings?

A. Pasteurella multocida
B. Vibrio haemolyticus
C. Vibrio cholera
D. Vibrio vulnificus

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Case 2: Post-transplant seizure

A 50-year-old man is brought to the emergency department after having an observed right-sided focal seizure. The patient received a living-donor kidney transplant 6 months ago for end-stage renal disease. Current medications are tacrolimus and mycophenolate mofetil. Family members noticed that he had poor memory and frequent confusion for several weeks before the seizure. The new kidney has been functioning well with no signs of rejection, infection or other complications. Medical history is otherwise unremarkable.

On physical examination, the patient is awake, alert and fully oriented, but has slowed verbal responses and difficulty finding appropriate words. Temperature is 36.7°C (98.1°F), pulse rate is 82/min, respiration rate is 18/min, and blood pressure is 154/96 mm Hg. No skin lesions are seen. Cardiopulmonary and abdominal examinations are normal. There is trace ankle edema. On neurologic examination, the cranial nerves are intact except for slight ptosis of the left eyelid. Deep tendon reflexes are more active on the right, and the plantar response is extensor bilaterally. Sensation is unimpaired. Laboratory studies on admission were as follows: hemoglobin level, 13.8 g/dL (138 g/L); hematocrit, 42%; leukocyte count, 6800/µL (6.8 × 109/L); platelet count, 185,000/µL (185 × 109/L); blood urea nitrogen, 33 mg/dL (11.78 mmol/L); serum creatinine, 1.6 mg/dL (141.47 µmol/L); serum electrolytes, normal; liver chemistry studies, normal; and urinalysis, 150 mg/dL protein, otherwise normal.

MRI of the brain shows lesions in cortical areas of the left temporoparietal lobes and scattered throughout the deep white matter.

Which of the following is the most likely cause of this patient's neurologic disease?

A. Polyomavirus BK
B. Polyomavirus JC
C. Herpes simplex virus encephalitis
D. Cerebrovascular accident

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Case 3: Fever in day care worker

A previously healthy 32-year-old woman, who works in a children's day care center, is brought to the emergency department in August because of a 2-day history of fever and chills and a 1-day history of a rash. The United States has been on a Code Red advisory alert (high threat) for the past 10 days. The patient's husband and two children are well.

On physical examination, the patient appears ill. Temperature is 38.7°C (101.7°F), pulse rate is 100/min, respiration rate is 18/min, and blood pressure is 144/94 mm Hg. Papules and crops of vesicles are present on the patient's trunk. Papular lesions and a few vesicles are also noted on her lower extremities with sparing of her palms and soles. The papules and vesicles appear in varying stages, ranging from new lesions to crusted lesions. No other skin or mucous membrane changes are noted, and general examination is unremarkable.

Which of the following is the most likely diagnosis?

A. Coxsackievirus infection
B. Ebola virus infection
C. Smallpox
D. Varicella (chickenpox)
E. Monkeypox

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Answers and commentary

Case 1

Correct answer: D. Vibrio vulnificus.

This patient has rapidly progressing necrotizing fasciitis resulting from Vibrio vulnificus infection of a skin abrasion acquired from an oyster shell. This organism is found in warm waters throughout the world and is responsible for many serious infections in the United States each year as a result of exposure to fish and seawater. V. vulnificus has now been reported as far north as Alaska but is much more common in warmer climates. Ingestion of seafood containing the organism can also cause sepsis via the gastrointestinal tract, especially in patients with cirrhosis or other liver diseases.

Pasteurella multocida infection occurs after animal bites and is much less likely to cause such severe necrotizing fasciitis with hemorrhagic bullae. Vibrio haemolyticus and Vibrio cholerae do not cause necrotizing fasciitis, although they may cause serious gastrointestinal and systemic disease.

Key points

  • Vibrio vulnificus infections are associated with exposure to seawater and fish.
  • Wound infections caused by Vibrio vulnificus may result in necrotizing fasciitis.

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Case 2

Correct answer: B. Polyomavirus JC.

The patient's findings are typical of progressive multifocal leukoencephalopathy, which is a demyelinating disease with a high mortality rate. It is associated with reactivation of polyomavirus JC and occurs most often in immunosuppressed patients. There is no treatment. However, if the disease is detected early, it can possibly be arrested by decreasing or stopping a patient's immunosuppressive agents. Therefore, if this patient's central nervous system damage is not too severe, he and his family should be told about the possibility of stopping immunosuppression and sacrificing the renal transplant to preserve remaining nervous system function.

Polyomavirus BK causes nephropathy and deteriorating renal function rather than progressive multifocal leukoencephalopathy. Herpes simplex virus encephalitis also is not known to cause lesions resembling progressive multifocal leukoencephalopathy. A cerebrovascular accident would not cause the scattered lesions in the brain that were seen on this patient's MRI scans.

Key points

  • Progressive multifocal leukoencephalopathy is a demyelinating disease with a high mortality rate that occurs primarily in immunosuppressed patients.
  • No treatment is available for patients with progressive multifocal leukoencephalopathy.

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Case 3

Correct answer: D. Varicella (chickenpox).

This patient most likely has varicella (chickenpox), based on her clinical presentation and her close contact with children. Although smallpox cannot always be easily differentiated from chickenpox, lesions due to varicella virus typically begin on the trunk, are more prominent on the trunk than on the extremities, and appear in various stages at the same time (new lesions followed by crusted lesions). Smallpox lesions are more prominent on the extremities, involve the palms and soles, and are all at the same stage (new lesions and crusted lesions seen simultaneously). Fever and rash occur simultaneously in patients with varicella, whereas patients with smallpox first develop fever that tends to decrease after the rash emerges.

The rash in patients with coxsackievirus infection is typically vesicular and involves the hands, feet and mouth. Ebola virus is associated with a petechial rash. Monkeypox is also difficult to differentiate from smallpox and varicella. The rash in monkeypox is centrifugal in distribution and at times is associated with a hemorrhagic component. However, 55% of patients with monkeypox develop lymphadenopathy, which seldom occurs in patients with smallpox or varicella.

Key points

  • The rash of smallpox is most prominent on the face and extremities, and lesions are all at the same stage of development.
  • The rash of chickenpox is most prominent on the trunk, with sparing of the palms and soles, and lesions are at different stages of development.