## **Core Concept**
The question tests the knowledge of vitamin deficiencies and their effects on metabolic pathways, specifically regarding the excretion of xanthurenic acid in urine. Xanthurenic acid is a metabolite related to tryptophan metabolism. The correct answer involves identifying the vitamin that, when deficient, leads to increased excretion of xanthurenic acid.

## **Why the Correct Answer is Right**
The correct answer, **C. Vitamin B6 (Pyridoxine)**, is related to tryptophan metabolism. Vitamin B6 is a co-factor for the enzyme kynureninase, which is involved in the breakdown of tryptophan. A deficiency in Vitamin B6 leads to a shunt in tryptophan metabolism, favoring the production of kynurenine and subsequently xanthurenic acid. Therefore, a deficiency in Vitamin B6 results in increased excretion of xanthurenic acid in the urine.

## **Why Each Wrong Option is Incorrect**
- **Option A: Vitamin B12** - This vitamin is crucial for the metabolism of fatty acids and amino acids, and its deficiency leads to megaloblastic anemia. It is not directly related to xanthurenic acid excretion.
- **Option B: Vitamin B2 (Riboflavin)**** - While important for energy production and can affect metabolism broadly, it is not specifically linked to the excretion of xanthurenic acid.
- **Option D: Folic Acid** - Folic acid deficiency also leads to megaloblastic anemia and is involved in numerous metabolic reactions but is not directly associated with increased xanthurenic acid excretion.

## **Clinical Pearl / High-Yield Fact**
A key clinical point to remember is that Vitamin B6 deficiency can lead to neurological symptoms and is also associated with increased excretion of xanthurenic acid in urine, which can be a diagnostic clue. This vitamin plays a critical role in many enzymatic reactions, and its deficiency can have wide-ranging effects on metabolism and neurological function.

## **Correct Answer: C. Vitamin B6 (Pyridoxine)**