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 child has cystic fibrosis. His parents repo a history of recurrent respiratory tract infections and bulky, foul-smelling stools. After assessment of his respiratory tract illness, the practitioner should also look for signs of which vitamin deficiency. 
Select and explain. 
A. Vitamin C, B. Vitamin A, C. Vitamin B1, D. Vitamin B6
B: Vitamin A
Answer: B: Vitamin A 
Explanation: Vitamin A