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?

Explanation

## Pathophysiology of Night Blindness in Vitamin A Deficiency ### Role of Vitamin A in Vision **Key Point:** Vitamin A (retinol) is converted to 11-cis-retinal, which is the chromophore of rhodopsin — the visual pigment essential for low-light vision in rod cells. 1. Retinol → Retinal (via retinol dehydrogenase) 2. Retinal → 11-cis-Retinal (via isomerase) 3. 11-cis-Retinal + Opsin → **Rhodopsin** (visual pigment) When vitamin A is deficient, 11-cis-retinal synthesis is impaired, leading to: - Reduced rhodopsin regeneration - Loss of rod cell function - **Night blindness** (earliest clinical sign of vitamin A deficiency) ### Clinical Findings in This Case | Feature | Mechanism | |---------|----------| | Night blindness | Inadequate 11-cis-retinal → defective rhodopsin | | Bitot spots | Foamy, triangular patches on conjunctiva; pathognomonic for vitamin A deficiency | | Vision loss | Progression to corneal scarring (keratomalacia) if untreated | | Low serum retinol | Confirms nutritional deficiency (alcohol impairs absorption and hepatic storage) | ### Why Alcohol Use Disorder Increases Risk **Clinical Pearl:** Chronic alcohol use impairs: - Intestinal absorption of vitamin A - Hepatic storage (liver is the main storage organ) - Conversion of retinol to retinal (aldehyde dehydrogenase is inhibited) - Nutritional intake (poor diet) **High-Yield:** Night blindness is the **earliest reversible sign** of vitamin A deficiency. If untreated, progression to xerophthalmia (corneal drying) and corneal ulceration occurs, leading to irreversible blindness. ### Mnemonic for Vitamin A Deficiency Manifestations **ABCDE:** - **A**nemia (impaired hematopoiesis) - **B**itot spots (conjunctival xerosis) - **C**orneal scarring (keratomalacia) - **D**ermatitis (follicular hyperkeratosis) - **E**ye signs (night blindness, xerophthalmia) **Tip:** In NEET PG, when a patient with poor nutrition/alcohol use presents with night blindness, always think vitamin A deficiency first — it is the most common cause of preventable blindness worldwide.