The active form of vitamin A is:

Ans. B. Retinol. (Ref. Harper's Biochem 27th/Ch. 44; Harrison 18th/Ch. 74; Robbin's fig.9-22).Vitamin A, in the strictest sense, refers to retinol. However, the oxidized metabolites, retinaldehyde and retinoic acid, are also biologically active compounds. The term retinoids includes all molecules (including synthetic molecules) that are chemically related to retinol. Retinaldehyde (11-cis) is essential form of vit A that is required for normal vision, whereas retinoic acid is necessary for morphogenesis, growth, and cell differentiation. Retinoic acid does not function in vision and, in contrast to retinol, is not involved in reproduction. Vitamin A also plays a role in iron utilization, humoral immunity, T cell-mediated immunity, NK cell activity, and phagocytosis. Vitamin A is commercially available in esterified forms (e.g., acetate, palmitate) since it is more stable as an ester.Retinol, perhaps the most important form of vitamin A, is the transport form and, as the retinol ester, also the storage form. It is oxidized in vivo to the aldehyde retinal (the form used in visual pigment) and the acid retinoic acid. Important dietary sources of vitamin A are animal derived (e.g., liver, fish, eggs, milk, butter). Yellow and leafy green vegetables such as carrots, squash, and spinach supply large amounts of carotenoids, many of which are provitamins that can be metabolized to active vitamin A in vivo; the most important of these is beta-carotene. Retinoids, refers to both natural and synthetic chemicals that are structurally related to vitamin A but do not necessarily have vitamin A activity.As with all fats, the digestion and absorption of carotenes and retinoids require bile, pancreatic enzymes, and some level of antioxidant activity in the food. Retinol, whether derived from ingested esters or from beta-carotene (through an intermediate oxidation step involving retinal), is transported in chylomicrons to the liver for esterification and storage. More than 90% of the body's vitamin A reserves are stored in the liver, predominantly in the perisinusoidal stellate (Ito) cells. In normal persons who consume an adequate diet, these reserves are sufficient for at least 6 months' deprivation. Retinoic acid, on the other hand, can be absorbed unchanged; it represents a small fraction of vitamin A in the blood and is active in epithelial differentiation and growth but not in the maintenance of vision.In the retina, retinaldehyde functions as the prosthetic group of the light-sensitive opsin proteins, forming rhodopsin (in rods) and iodopsin (in cones). Any one cone cell contains only one type of opsin, and is sensitive to only one color. In the pigment epithelium of the retina, all-trans-retinol is isomerized to 11-cis-retinol and oxidized to 11-cis-retinaldehyde. This reacts with a lysine residue in opsin, forming the holoprotein rhodopsin. The absorption of light by rhodopsin causes isomerization of the retinaldehyde from 11-cis to all-trans, and a conformational change in opsin. This results in the release of retinaldehyde from the protein, and the initiation of a nerve impulse. The formation of the initial excited form of rhodopsin, bathorhodopsin, occurs within picoseconds of illumination. There is then a series of conformational changes leading to the formation of metarhodopsin II, which initiates a guanine nucleotide amplification cascade and then a nerve impulse. The final step is hydrolysis to release all-trans-retinaldehyde and opsin. The key to initiation of the visual cycle is the availability of 11-cis-retinaldehyde, and hence vitamin A. In deficiency, both the time taken to adapt to darkness and the ability to see in poor light are impaired.VitaminsVitaminActive derivative or co-factor formClinical FindingDietary Level per Day Asso with Overt Deficiency in AdultsContributing Factors to DeficiencyThiamineThiamine pyrophosphateBeriberi; neuropathy, muscle weak- ness and wasting, cardiomegaly, edema, ophthalmoplegia, confabulation<0.3 mg/1000 kcalAlcoholism, chronic diuretic use, hyperemesisRiboflavinFMN and FADMagenta tongue, angular stomatitis, seborrhea, cheilosis<0.6 mg--NiacinNADp and NADPellagra: pigmented rash of sun- exposed areas, bright red tongue, diarrhea, apathy, memory loss, disorientation<9.0 niacin equivalentsAlcoholism, vitamin B6 deficiency, riboflavin deficiency, tryptophan deficiencyVitamin B6Pyridoxal phosphateSeborrhea, glossitis, convulsions, neuropathy, depression, confusion, microcytic anemia<0.2 mgAlcoholism, isoniazidFolatePolyglutamte forms of trtrahydrofolate with C-unitMegaloblastic anemia, atrophic glossitis, depression, homocysteine<100 mcg/dAlcoholism, sulfasalazine, pyrimethamine, triamtereneVitamin B12Methylcobalamine AdenosylcobalaminMegaloblastic anemia, loss of vibratory and position sense, abnormal gait, dementia, impotence, loss of bladder and bowel control, homocysteine, methylmalonic acid<1.0 mcg/dGastric atrophy (pernicious anemia), terminal ileal disease, strict vegetarianism, acid-reducing drugs (e.g., H2 blockers)Vitamin CAscorobic acid and dehydroascorbic acidScurvy: petechiae, ecchymosis, coiled hairs, inflamed and bleeding gums, joint effusion, poor wound healing, fatigue<10 mg/dSmoking, alcoholismVitamin ARetinol, retinal- dehyde, and retinoic acidXerophthalmia, night blindness,Bitot's spots, follicular hyperkeratosis, impaired embryonic development, immune dysfunction<300 mcg/dFat malabsorption, infection, measles, alcoholism, protein- energy malnutritionVitamin D1,25-Dihydroxy- vitamin DRickets: skeletal deformation, rachitic rosary, bowed legs; osteomalacia<2.0 mcg/dAging, lack of sunlight exposure, fat malabsorption, deeply pigmented skinVitamin ETocopherols and tocotrienolsPeripheral neuropathy, spinocere- bellar ataxia, skeletal muscle atrophy, retinopathyNot described unless underlying contributing factor is presentOccurs only with fat malabsorption or genetic abnormalities of vitamin E metabolism/ transportVitamin KVitamin K hydroquinone useElevated prothrombin time, bleeding<10 mcg/dFat malabsorption, liver disease, antibiotic