Here is the answer to this weeks #USMLE review question: Choice d is correct. Megacolon can be caused by a number of infectious organisms, including Clostridium difficile and Entamoeba histolytica. However, the combination of megacolon and cardiac symptoms strongly points to Trypanosoma cruzi, which can cause dilated cardiomyopathy, toxic megacolon, and achalasia. This protozoan, spread by the reduviid bug, can be diagnosed by visualizing the organism on blood smear. Other techniques include serology and xenodiagnosis (allowing reduviid bugs to bite the patient, then seeing whether they develop an organism burden). Choice a is incorrect. The adhesive tape test, or Scotch® tape test, is used to obtain Enterobius vermicularis eggs for microscopic examination. This organism causes perianal itching and potentially several other symptoms, but none as severe as those seen in this patient. Choice b is incorrect. T. cruzi can uncommonly cause neurologic disease, the most severe being meningoencephalitis. However, the organism would not appear on cytologic examination of cerebrospinal fluid. Choice c is incorrect. While T. cruzi causes inflammation of the myenteric plexus, which can be observed on colonic tissue biopsy, the organisms themselves are rarely visible. Choice e is incorrect. Several pathogenic organisms (or their eggs) that cause gastrointestinal symptoms can be observed via a stool ova and parasites examination. However, T. cruzi does not involve the lumenal surface of the colon and would not be present in a patients feces.
Posted on: Thu, 24 Jul 2014 15:00:00 +0000
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