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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.


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

Diagnosis: Infectious mononucleosis is 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 Chapter 37 for additional information.


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 Chapter 50 for additional information.


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 additional information on Giardia, Cryptosporidium, and Entamoeba in Chapter 51.


A 35-year-old man who is human immunodeficiency virus (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 that 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 Chapter 50 for additional information. If acid-fast rods are seen in spinal fluid, think Mycobacterium tuberculosis. See Chapter 21 for additional information.


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 ...

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