**Question:** Allopurinol prevents conversion of -

A. xanthine dehydrogenase to xanthine oxidase
B. xanthine dehydrogenase to xanthine oxidase
C. xanthine oxidase to hypoxanthine
D. uric acid to xanthine

**Core Concept:** Allopurinol is an inhibitor of xanthine oxidase enzyme. Xanthine oxidase catalyzes the final step in purine catabolism, which results in the formation of uric acid from xanthine and hypoxanthine.

**Why the Correct Answer is Right:** Allopurinol is a xanthine oxidase inhibitor, meaning it specifically blocks the enzyme responsible for converting xanthine dehydrogenase to xanthine oxidase. This inhibition leads to a decrease in uric acid production and thus reduces hyperuricemia, which is a common feature in conditions such as gout.

**Why Each Wrong Option is Incorrect:**

A. Xanthine dehydrogenase to xanthine oxidase: Allopurinol prevents this conversion, making option A incorrect.
B. Xanthine dehydrogenase to xanthine oxidase: Similar to option A, allopurinol also prevents this conversion, making option B incorrect.
C. Xanthine oxidase to hypoxanthine: This is a downstream product of xanthine oxidase, and allopurinol does not target this step. This option is incorrect.
D. Uric acid to xanthine: Allopurinol has no effect on this step, as it does not involve xanthine oxidase and is part of the uric acid synthesis pathway from hypoxanthine. This option is incorrect.

**Clinical Pearl:** Allopurinol is a commonly used medication to treat hyperuricemia and gout by inhibiting xanthine oxidase. It is crucial to understand the correct inhibition targets of allopurinol to effectively prescribe and explain its mechanism of action in clinical practice.