USMLE-Rx Step 1 Practice Q's

USMLE-Rx Step 1 Qmax Challenge #4057

Check out today’s Step 1 Qmax Question Challenge.

Know the answer? Post it below! Don’t forget to check back for an update with the correct answer and explanation (we’ll post it in the comments section below).

USMLE-Rx Step 1 Qmax Challenge #4057After a night of heavy drinking, a 37-year-old man experiences pain, warmth, swelling, and redness around his big toe, shown radiographically in the image. He had been previously warned against drinking heavily, as it could exacerbate the pain in his toe.

Which of the following enzymes is responsible for the final conversion of the product responsible for this patient’s pain?

A. Adenosine deaminase
B. Cystathionine synthase
C. Hypoxanthine guanine phosphoribosyltransferase
D. Phenylalanine hydroxylase
E. Xanthine oxidase


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8 replies »

  1. The correct answer is E. This patient has gout, which is characterized by precipitation of monosodium urate crystals in the joints. Xanthine is converted into uric acid by xanthine oxidase, the final step of the purine salvage pathway. Allopurinol, a medication used to prevent gout attacks, inhibits xanthine oxidase. Acute gout attacks are instead treated with nonsteroidal anti-inflammatory drugs (NSAIDs) like indomethacin. NSAIDs are useful in the acute setting because they treat pain and inflammation, which are the primary issue in an acute attack.

    A is not correct. Adenosine deaminase converts adenosine to inosine, another step in the purine pathway. Deficiency of this enzyme can cause severe combined immunodeficiency disease.

    B is not correct. Cystathionine synthase converts homocysteine to cystathionine with the help of vitamin B6. It is not in the purine salvage pathway and its deficiency leads to homocystinuria. Homocystinuria is treated by decreasing methionine and increasing cysteine in the diet.

    C is not correct. Hypoxanthine guanine phosphoribosyl transferase deficiency is associated with Lesch-Nyhan syndrome. This enzyme converts hypoxanthine to inosine monophosphate and guanine to guanosine monophosphate, but is not the final step in the pathway.

    D is not correct. Phenylalanine hydroxylase converts phenylalanine to tyrosine and is not in the purine salvage pathway. Its deficiency leads to phenylketonuria, in which tyrosine becomes essential to the diet. Treat with decreased phenylalanine in the diet and increase tyrosine consumption.


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