Night blindness may be caused by all except
**Core Concept**
Night blindness, also known as nyctalopia, is a condition characterized by difficulty seeing in low light environments. This is often due to impaired rhodopsin regeneration or reduced rod photoreceptor function.
**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of night blindness. Rhodopsin, a protein found in rod cells, is essential for night vision. In conditions affecting rhodopsin regeneration or rod cell function, night blindness occurs. For instance, Vitamin A deficiency impairs rhodopsin regeneration, leading to night blindness. Similarly, conditions like retinitis pigmentosa affect rod cell function, causing night blindness.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is a potential cause of night blindness. Vitamin A deficiency impairs rhodopsin regeneration, leading to night blindness.
**Option B:** This option is also a potential cause of night blindness. Retinitis pigmentosa affects rod cell function, causing night blindness.
**Option C:** This option is not a cause of night blindness. In fact, it's a condition that affects the peripheral vision and is characterized by a loss of peripheral vision. It's not related to the pathophysiology of night blindness.
**Clinical Pearl / High-Yield Fact**
One important fact to remember is that Vitamin A deficiency can cause night blindness, and it's essential to diagnose and treat this deficiency promptly to prevent long-term vision impairment.
**Correct Answer: C. Usher syndrome**