Vitamin A


 * 1) the original vitamin A, now known as retinol
 * 2) any *-ionone derivative, except provitamin A carotenoids, possessing qualitatively the biological activity of retinol; deficiency interfering with the production and resynthesis of rhodopsin, thereby causing night blindness, and producing a keratinizing metaplasia of epithelial cells that may result in xerophthalmia, keratosis, susceptibility to infections, and retarded growth