Allopurinol specifically inhibits
**Core Concept**
Allopurinol is a medication used to treat hyperuricemia and gout. It specifically inhibits an enzyme involved in purine metabolism, which is a key process in the production of uric acid.
**Why the Correct Answer is Right**
Allopurinol works by inhibiting xanthine oxidase, an enzyme responsible for the conversion of hypoxanthine to xanthine and xanthine to uric acid. This enzyme is crucial in the production of uric acid, and by inhibiting it, allopurinol reduces the formation of uric acid in the body. As a result, allopurinol is effective in lowering uric acid levels and preventing gout attacks.
**Why Each Wrong Option is Incorrect**
**Option A:** Xanthine oxidase is the correct target of allopurinol, so this option is incorrect because it is the correct answer.
**Option B:** While allopurinol does affect purine metabolism, it does not specifically inhibit hypoxanthine-guanine phosphoribosyltransferase (HGPRT), an enzyme involved in purine salvage pathways.
**Option C:** Allopurinol does not specifically inhibit uric acid transporters, such as the urate transporter 1 (URAT1), which are involved in the renal reabsorption of uric acid.
**Clinical Pearl / High-Yield Fact**
It's essential to note that allopurinol can increase the risk of xanthine stones in patients with kidney disease, so regular monitoring of kidney function is crucial in these patients.
**Correct Answer:** A. Xanthine oxidase