Deficiency of Vitamin A primarily affects the visual cycle by impairing the formation of rhodopsin, a photopigment in the rod cells of the retina crucial for low-light vision. Without adequate vitamin A, the synthesis of rhodopsin is reduced, leading to difficulty seeing in dim light, clinically manifested as night blindness. This symptom is an early and specific sign of vitamin A deficiency, making it a critical indicator for diagnosis and treatment. Correcting the deficiency restores rhodopsin production and improves night vision.

Reference: Harrison's Principles of Internal Medicine, Kasper et al., 20th Edition.