This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
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: 
Select and explain. 
A. Vitamin C, B. Vitamin B1, C. Vitamin B2, D. Vitamin B6
D: Vitamin B6
Answer: D: Vitamin B6 
Explanation: Pyridoxine deficiency is associated with neurological symptoms such as depression, irritability, nervousness and mental confusion. Convulsions and peripheral neuropathy are observed in severe deficiency.
The synthesis of niacin coenzymes (NAD+ and  NADP+)  from tryptophan is impaired. Xanthurenic acid, produced in high quantities is excreted in urine, which serves as a reliable index (particularly after tryptophan load test) for B6 deficiency.

Satyanarayana- Biochemistry, 3rd edition, pg-146