Allopurinol prevents conversion of : a) Hypoxanthine to xanthine b) Xanthine to hypoxanthine c) Hypoxanthine to I.M.P. d) Xanthine to uric acid

**Core Concept**
Allopurinol is a xanthine oxidase inhibitor, which plays a crucial role in the management of gout and hyperuricemia. It prevents the conversion of hypoxanthine to xanthine and xanthine to uric acid, thereby reducing uric acid production in the body.

**Why the Correct Answer is Right**
Allopurinol works by inhibiting the enzyme xanthine oxidase, which is responsible for the conversion of hypoxanthine to xanthine and xanthine to uric acid. This enzyme is a molybdenum-flavoenzyme that catalyzes the oxidation of hypoxanthine to xanthine and xanthine to uric acid. By inhibiting this enzyme, allopurinol reduces the production of uric acid, which is a major contributor to gout and hyperuricemia.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because allopurinol actually inhibits the conversion of hypoxanthine to xanthine, not the other way around.

**Option B:** This option is incorrect because allopurinol does not inhibit the conversion of xanthine to hypoxanthine; instead, it inhibits the conversion of xanthine to uric acid.

**Option C:** This option is incorrect because allopurinol does not affect the conversion of hypoxanthine to inosine monophosphate (IMP); this conversion is catalyzed by the enzyme hypoxanthine-guanine phosphoribosyltransferase (HGPRT).

**Clinical Pearl / High-Yield Fact**
A key point to remember is that allopurinol can also reduce the production of uric acid by inhibiting the conversion of xanthine to uric acid. Additionally, allopurinol can be used in combination with uricosuric agents to enhance its effectiveness in managing gout and hyperuricemia.

**Correct Answer:** D. Uric acid.