**Question:** A 30 year old female patient complains of irritability and nervousness. Patient gives history of consumption of oral contraceptives. Lab investigation reveals presence of xanthurenic acid in urine. This patient may be suffering from deficiency of:

A. Vitamin B6
B. Vitamin B3
C. Vitamin B12
D. Vitamin B9 (Folate)

**Correct Answer:**

**Core Concept:**

Xanthurenic acid is a kynurenine pathway intermediate that is formed from the breakdown of tryptophan, an essential amino acid. The kynurenine pathway is crucial for the conversion of tryptophan to nicotinamide adenine dinucleotide (NAD+), a vital coenzyme for cellular respiration. This pathway involves the action of several enzymes, including kynurenine aminotransferase (KAT) and kynurenine 3-monooxygenase (KMO).

**Why the Correct Answer is Right:**

In this case, the presence of xanthurenic acid in urine indicates a deficiency of Vitamin B6 (pyridoxine). This is because Vitamin B6 is a cofactor for KAT and KMO enzymes involved in the kynurenine pathway. A deficiency of Vitamin B6 leads to a dysfunction in the enzymes' function, causing an accumulation of xanthurenic acid.

**Why Each Wrong Option is Incorrect:**

A. Vitamin B6 deficiency would not lead to the accumulation of xanthurenic acid, as it affects KAT enzymes.
B. Vitamin B3 (nicotinic acid) deficiency does not impact the kynurenine pathway or the enzymes involved, rendering this option incorrect.
C. Vitamin B12 (cobalamin) deficiency is unrelated to the kynurenine pathway or xanthurenic acid levels.
D. Vitamin B9 (folic acid) deficiency does not affect the kynurenine pathway or xanthurenic acid levels, making this option incorrect.

**Clinical Pearl:**

Vitamin B6 deficiency should be suspected in patients presenting with neurological symptoms like irritability, nervousness, and cognitive dysfunction, especially if they take oral contraceptives that can interfere with vitamin B6 absorption. Diagnosing vitamin B6 deficiency can be confirmed by measuring xanthurenic acid levels in urine.