**Core Concept**
Gout is a form of inflammatory arthritis characterized by the deposition of monosodium urate (MSU) crystals in joints. This condition is primarily caused by an abnormality in uric acid metabolism, leading to hyperuricemia.

**Why the Correct Answer is Right**
The defective enzyme responsible for gout is xanthine oxidase, which catalyzes the final two steps of purine catabolism, converting hypoxanthine to xanthine and xanthine to uric acid. In gout, the overactivity of xanthine oxidase results in excessive production of uric acid, leading to its accumulation and eventual crystallization in joints.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as it does not relate to the pathogenesis of gout.
* **Option B:** While allopurinol is a medication used to treat gout by inhibiting xanthine oxidase, it is not the defective enzyme itself.
* **Option C:** This option is incorrect as it refers to a different metabolic pathway unrelated to gout.

**Clinical Pearl / High-Yield Fact**
It's essential to note that not all cases of gout are due to overactivity of xanthine oxidase; some may be caused by genetic defects in uric acid transport or other mechanisms. A high index of suspicion is required for accurate diagnosis.

**Correct Answer:** C. Xanthine oxidase