Which of the following conditions can be reversed with vitamin A?

Nyctalopia, or night blindness, is a condition that occurs due to a deficiency of vitamin A, specifically affecting the ability to see in low light or darkness. Vitamin A plays a crucial role in the production of rhodopsin, a pigment found in the retina that is essential for night vision. When a person experiences a deficiency in vitamin A, the synthesis of rhodopsin is impaired, leading to difficulties in seeing in dimly lit environments.

The restoration of adequate vitamin A levels can significantly improve or completely reverse the effects of nyctalopia. In contrast, xerophthalmia, while it is also related to vitamin A deficiency and impacts eye health, encompasses a more severe range of eye conditions, which include dryness of the conjunctiva and cornea and eventually can lead to blindness.

Conditions such as Wilson's disease, which involves copper metabolism, and homocysteinuria, which involves the metabolism of the amino acid homocysteine, are unrelated to vitamin A deficiency and its supplementation does not have any impact on these diseases.

Thus, the correct understanding of nyctalopia being directly reversed with vitamin A supplementation highlights the importance of this nutrient in maintaining proper vision, particularly in low-light conditions.