This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Vitamins and Minerals, Subject name: Biochemistry
Deficiency of which of the following vitamins is associated with progressive keratinization of the cornea? 
Select and explain. 
A. Vitamin A, B. Vitamin C, C. Niacin, D. Pyridoxine
A: Vitamin A
Answer: A: Vitamin A 
Explanation: Vitamin A deficiency is the most impoant preventable cause of blindness. The earliest sign of deficiency is a loss of sensitivity to green light-followed by impairment to adapt to dim light-then night blindness (inability to see in the dark) More prolonged deficiency leads to xerophthalmia: keratinization of the cornea, and blindness