A 58-year-old man with a 20-year history of alcohol use disorder presents with progressive vision loss, night blindness, and bitot spots on conjunctival examination. His dietary intake is poor, consisting mainly of refined carbohydrates and alcohol. Serum retinol level is 15 µg/dL (normal: 30–100 µg/dL). Which of the following best explains the pathophysiology of his night blindness?