Vitamin A (retinol) is a fat-soluble vitamin essential for vision, immune function, cell growth, and embryonic development. This test measures vitamin A levels in the blood to evaluate nutritional status and diagnose deficiency or toxicity. Vitamin A is crucial for the visual cycle, particularly for night vision, as it forms the light-absorbing component of rhodopsin in retinal receptors. Deficiency can lead to night blindness, xerophthalmia (corneal damage), increased infection susceptibility, and impaired growth. Excessive vitamin A intake can cause toxicity with symptoms including nausea, headache, liver damage, and in pregnant women, birth defects. This test is particularly important for evaluating patients with vision problems, assessing nutritional status in malnourished individuals, monitoring patients on vitamin A supplementation, and investigating suspected vitamin A toxicity.