Night blindness is an early symptom of vitamin A deficiency, a condition that impairs the eye’s ability to adapt to low light conditions. Vitamin A plays a crucial role in the visual cycle, specifically in the formation of visual pigments in the retina. The patient’s failure to improve with dietary supplementation and vitamin A injections suggests an issue beyond simple vitamin A deficiency—likely a defect in the enzymatic conversion of vitamin A into forms usable for vision.

Vitamin A Metabolism and Vision

Vitamin A undergoes several enzymatic reactions to convert into retinal, the aldehyde form essential in the visual cycle:

• Retinol dehydrogenase (A): Converts retinol (alcohol form of vitamin A) to retinal (aldehyde form).

• Retinal isomerase (C): Specifically converts all-trans-retinal to 11-cis-retinal, the isomer that binds to opsin proteins in photoreceptor cells to form rhodopsin, which is essential for low-light vision.

• Retinal reductase (B): Converts retinal back to retinol, mainly involved in recycling but not crucial for initial visual pigment formation.

• Retinal synthase (D): The term is less commonly used and not a key enzyme in the visual cycle.

Understanding the Cause of Night Blindness in This Case

The lack of improvement after direct vitamin A injection suggests that the patient’s cells are unable to convert vitamin A into the active visual pigments needed for vision.

• A defect in retinol dehydrogenase (A) would impair conversion of retinol to retinal.

• A defect in retinal isomerase (C) would prevent formation of 11-cis-retinal, disrupting rhodopsin synthesis.

• Retinal reductase (B) mainly helps recycling retinal; its deficiency alone is less likely to cause night blindness.

• Retinal synthase (D) is not a recognized, critical enzyme for this process.

Since the question options pair B and C, but B (retinal reductase) is less directly linked to night blindness, the most likely cause includes defects in:

(4) C and D both

But since D is less relevant, and A (retinol dehydrogenase) is a key enzyme, the best answer from the choices is:

(1) A only