retinol

Retinol is a fat-soluble vitamin (Vitamin A) stored in the liver and transported in blood bound to retinol-binding protein (RBP). It is converted to retinal, which combines with opsin to form rhodopsin in rod photoreceptors, critical for dim-light (night) vision via the visual cycle. Retinoic acid, an active metabolite, regulates gene transcription through nuclear retinoic acid receptors (RAR/RXR), controlling epithelial cell differentiation and maintaining mucous-secreting epithelium (anti-oxidant/anti-infective properties). Deficiency (common in fat malabsorption states, e.g., cystic fibrosis, or in developing countries) presents with night blindness, xerophthalmia, Bitot spots, corneal ulceration, and increased susceptibility to infections (measles-related mortality). Excess intake (hypervitaminosis A) causes toxicity: dry skin, alopecia, hepatotoxicity, pseudotumor cerebri (headache, papilledema), and teratogenic effects (craniofacial and CNS malformations)—critical for USMLE association with isotretinoin (a retinoic acid derivative used for severe acne, contraindicated in pregnancy due to teratogenicity). Retinol also plays a role in bone remodeling and reproduction. Clinical pearl: Vitamin A toxicity mnemonic - 'Vitamin A toxicity = dry, cracked skin + pseudotumor cerebri + teratogenic (like isotretinoin).'