**Core Concept**
Vitamin B6 (pyridoxine) plays a crucial role in the metabolism of tryptophan, an essential amino acid. A deficiency in vitamin B6 can lead to impaired tryptophan metabolism, resulting in the accumulation of abnormal metabolites.

**Why the Correct Answer is Right**
Vitamin B6 is necessary for the conversion of tryptophan to niacin and other downstream products. A deficiency in vitamin B6 leads to a block in this pathway, causing tryptophan to be shunted towards the kynurenine pathway. This results in the accumulation of xanthurenic acid, a metabolite that is not normally produced in significant quantities. Xanthurenic acid is excreted in the urine, serving as a diagnostic marker for vitamin B6 deficiency.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because vitamin B12 deficiency primarily affects the metabolism of fatty acids and amino acids, but not tryptophan metabolism directly.

**Option B:** This option is incorrect because vitamin C deficiency affects collagen synthesis and iron absorption, but not tryptophan metabolism.

**Option C:** This option is incorrect because vitamin E deficiency affects cell membrane integrity and antioxidant defenses, but not tryptophan metabolism.

**Clinical Pearl / High-Yield Fact**
Xanthurenic acid is a useful diagnostic marker for vitamin B6 deficiency, particularly in patients with a history of tryptophan-rich diets or those with impaired renal function.

**Correct Answer:** C.