**Core Concept**
Nyctalopia, or night blindness, results from impaired vision in low light due to deficiency of vitamin A, which is essential for the synthesis of rhodopsin, a photopigment in the retina responsible for scotopic (night) vision.

**Why the Correct Answer is Right**
Vitamin A is a precursor for retinaldehyde, which is converted to rhodopsin in rod cells of the retina. Rhodopsin enables vision in dim light. Deficiency of vitamin A leads to reduced rhodopsin production, impairing night vision. This deficiency is particularly common in malnourished populations and is a hallmark of vitamin A deficiency disease.

**Why Each Wrong Option is Incorrect**
Option B: Vitamin B complex includes B1, B2, B6, B12, etc., which are involved in energy metabolism and red blood cell formation, not photoreception. Deficiency causes anemia or neuropathy, not nyctalopia.
Option C: Vitamin C is an antioxidant and involved in collagen synthesis; deficiency causes scurvy, not night blindness.
Option D: Vitamin E is an antioxidant protecting cell membranes; deficiency causes neuromuscular disorders, not visual defects.

**Clinical Pearl / High-Yield Fact**
Vitamin A deficiency is the leading cause of preventable blindness worldwide. Early detection through nyctalopia screening can guide supplementation and improve outcomes in endemic areas.

✓ Correct Answer: A. Vitamin A