**Core Concept**
Purine metabolism is a complex process that involves the breakdown of purine nucleotides to their end products. In non-primate mammals, the end product of purine metabolism is different from that in humans and primates.

**Why the Correct Answer is Right**
In non-primate mammals, the end product of purine metabolism is uric acid is not accurate, as it is in humans and primates. Instead, non-primate mammals convert xanthine to allantoin, which is then excreted in the urine. This process is facilitated by the enzyme uricase, which is present in non-primate mammals but absent in humans and primates. Allantoin is a more soluble compound than uric acid, making it easier to excrete.

**Why Each Wrong Option is Incorrect**
**Option A:** Uric acid is the end product of purine metabolism in humans and primates, but not in non-primate mammals.
**Option B:** Xanthine is an intermediate in purine metabolism, but it is not the end product in non-primate mammals.
**Option C:** Hypoxanthine is another intermediate in purine metabolism, but it is not the end product in non-primate mammals.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the presence or absence of uricase enzyme is a key factor in determining the end product of purine metabolism in different species.

**Correct Answer: D. Allantoin**