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USMLE (United States Medical Licensing Examination) Practice Exam 05

Practice exam for USMLE (United States Medical Licensing Examination).

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Question 1 of 1000%
Time remaining120:00
Attempts allowedUnlimited
Difficulty mixE • M • H
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Question 1

MEDIUM

A 24-year-old man is evaluated for recurrent kidney stones. He reports passing several small stones since age 18. He has no history of urinary tract infections. Physical examination is unremarkable. Urinalysis shows hexagonal crystals. A cyanide-nitroprusside test is positive. Which of the following is the most likely underlying defect responsible for this patient’s condition?

Recurrent stones with hexagonal crystals and a positive cyanide-nitroprusside test indicate cystinuria due to defective proximal tubular (and intestinal) transport of dibasic amino acids (COLA: cystine, ornithine, lysine, arginine). Homocystinuria (CBS deficiency) causes thromboembolism and lens subluxation; oxalate stones are envelope/dumbbell shaped.

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