USMLE Answer 55
55. The correct answer is A. The patient has myocarditis due to Trypanosoma cruzi . This infectious condition, known as Chagas disease, is endemic in vast areas of South America and is transmitted from person to person by triatomids known as “kissing bugs.” Experts assess the number of persons with Chagas disease at about 7 million, with about 35 million at risk in South America. T. cruzi is an intracellular protozoon that localizes mainly in the heart and nerve cells of the myenteric plexus, leading to myocarditis and dysmotility of hollow organs, such the esophagus, colon, and ureter. Cardiac involvement manifests with ventricular dilatation and congestive heart failure secondary to myocyte necrosis and fibrosis. Intracellular parasites can be visualized in tissue sections. Chagas disease is a cause of acquired achalasia, in which the distal third of the esophagus dilates because of loss of its intrinsic innervation. A similar pathologic mechanism accounts for megacolon and megaureter in Chagas disease.
The remaining choices refer to different infectious conditions that may also involve the myocardium:
Chronic arthritis (choice B) is a manifestation of the chronic stage of Lyme disease, which is caused by Borrelia burgdorferi and is transmitted to humans by deer ticks. Skin, CNS, and heart are the main targets of this infection.
Cysts in the brain (cysticerci; choice C) may develop as a consequence of infestation by the tapeworm Taenia solium. Humans acquire this parasite by ingesting the eggs from undercooked pork. Cysticercosis may also affect the heart, skeletal muscle, and skin.
Group B coxsackievirus infections cause pleuritis (choice D) and myocarditis, manifesting with fever, chest pain, and, if myocarditis is severe, congestive heart failure. As in any form of viral myocarditis, the myocardium is infiltrated by lymphocytes, but there are no morphologic markers specific for Coxsackievirus infection.
Splenomegaly (choice E), often of massive proportions, is seen in patients with malaria. Plasmodium organisms can also invade the myocardium, leading to myocarditis.