**Core Concept**
The breakdown of purine nucleotides in humans involves a series of enzymatic reactions that ultimately yield a specific end product. This process is crucial for maintaining cellular homeostasis and preventing the accumulation of toxic compounds.

**Why the Correct Answer is Right**
In humans, the catabolism of purine nucleotides is a multi-step process that involves the action of several enzymes, including xanthine oxidase. This enzyme catalyzes the conversion of hypoxanthine to xanthine and then to uric acid. Uric acid is the main catabolic product of purine nucleotides in humans, as other species, such as primates, have uricase, an enzyme that breaks down uric acid further. In contrast, humans lack uricase, making uric acid the final product of purine catabolism. This unique aspect of human biochemistry is due to the loss of the uricase gene during evolution.

**Why Each Wrong Option is Incorrect**
**Option A:** Ammonia + CO2 is not a correct answer because while ammonia is a byproduct of purine catabolism, it is not the main catabolic product. Additionally, CO2 is a byproduct of various metabolic reactions, but it is not directly related to purine catabolism.

**Option B:** Ammonia is a byproduct of purine catabolism, but it is not the main catabolic product. The conversion of hypoxanthine to xanthine and then to uric acid is a more specific and accurate description of the process.

**Option D:** CO2 and Water are byproducts of many metabolic reactions, but they are not the main catabolic products of purine nucleotides.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the accumulation of uric acid can lead to hyperuricemia, which is associated with gout, a type of inflammatory arthritis. Understanding the unique aspects of human purine catabolism is crucial for diagnosing and managing this condition.

**✓ Correct Answer: C. Uric Acid**