vitamin A

Vitamin A exists in forms including retinol, retinal, and retinoic acid, and is derived from animal sources (preformed retinol) or plant carotenoids (provitamin A, e.g., beta-carotene). It is absorbed in the small intestine with dietary fat and stored in the liver (stellate cells). Retinal is a key component of rhodopsin in rod photoreceptors, essential for vision in low light; deficiency causes night blindness and can progress to xerophthalmia, Bitot spots, and corneal ulceration (keratomalacia), a leading cause of preventable blindness in developing countries. Retinoic acid acts as a ligand for nuclear retinoic acid receptors (RAR/RXR), regulating gene transcription important for epithelial cell differentiation, immune function, and mucus secretion; deficiency leads to squamous metaplasia (especially in respiratory and urinary epithelium), predisposing to infections and kidney stones. Vitamin A is also crucial for immune function, and deficiency increases susceptibility to measles and diarrheal disease, so WHO recommends supplementation in developing countries. Vitamin A toxicity (hypervitaminosis A) can occur with excessive supplementation and causes symptoms including alopecia, dry skin, hepatotoxicity, pseudotumor cerebri (headache, blurred vision, papilledema without mass lesion), and bone abnormalities. Vitamin A and its derivatives are highly teratogenic; retinoic acid derivatives like isotretinoin (used for severe acne) require strict pregnancy prevention programs due to risk of craniofacial, cardiac, and CNS malformations. All-trans retinoic acid (ATRA) is also used therapeutically as differentiation therapy in acute promyelocytic leukemia (APL, associated with PML-RARA fusion from t(15;17)), inducing terminal differentiation of the malignant promyelocytes.