Which among the following is the earliest sign of vitamin-A deficiency in developing countries?
**Core Concept**
Vitamin-A deficiency is a significant public health concern in developing countries, leading to various ocular and non-ocular manifestations. The earliest sign of vitamin-A deficiency is related to the critical role of this vitamin in maintaining the integrity and function of the epithelial lining of the eyes.
**Why the Correct Answer is Right**
The earliest sign of vitamin-A deficiency is night blindness (nyctalopia), which is caused by the impaired function of rhodopsin, the visual pigment in the retina. Rhodopsin is a complex of the protein opsin and the vitamin-A derivative 11-cis retinal. In the absence of sufficient vitamin A, the conversion of 11-cis retinal to its active form is impaired, leading to reduced sensitivity to light and difficulty adapting to darkness. This condition often presents as difficulty seeing in low light conditions, which can be a significant problem for individuals in areas with limited access to electricity.
**Why Each Wrong Option is Incorrect**
**Option A:** Corneal ulcers are a complication of vitamin-A deficiency, but they are not the earliest sign.
**Option B:** Xerophthalmia, a more severe form of vitamin-A deficiency, includes night blindness, but it is not the first manifestation.
**Option C:** Conjunctival xerosis, a feature of vitamin-A deficiency, is a later sign than night blindness.
**Clinical Pearl / High-Yield Fact**
Night blindness is a reversible condition if vitamin-A supplementation is initiated early in the course of deficiency. It is essential to recognize this early sign to prevent progression to more severe forms of vitamin-A deficiency.
**Correct Answer:** C. Night blindness.