MLT GlobeChemistry: TDM, toxicology & vitamins
109 Chemistry MCQs on TDM, toxicology & vitamins with answers and explanations.
Which patient group is at greatest risk of pyridoxine (vitamin B6) deficiency?
Alcohol's metabolite acetaldehyde speeds breakdown of pyridoxal phosphate, and poor diet adds to the deficit. Isoniazid therapy is another classic cause.
Dicoumarol and warfarin act as anticoagulants by opposing which vitamin?
Coumarins block vitamin K epoxide reductase, preventing γ-carboxylation of factors II, VII, IX and X. Menadione is a synthetic vitamin K.
Active vitamin D (calcitriol) is produced by sequential hydroxylation in which organs?
Liver 25-hydroxylase forms calcidiol; renal 1α-hydroxylase converts it to 1,25-dihydroxycholecalciferol. Renal failure therefore causes calcitriol deficiency.
An infant fed only cow's milk for a long period is most at risk of lacking which vitamin?
Milk is a good source of vitamin A, riboflavin and B12 but provides very little ascorbic acid; iron is also deficient.
Which best describes vitamins?
Vitamins are organic compounds required in trace amounts, often as coenzyme precursors, that the body cannot make adequately. They supply no significant energy.
Digoxin, a drug often monitored in the laboratory, belongs to which chemical class?
Digoxin is a steroid linked to sugar residues through glycosidic bonds, i.e. a cardiac glycoside from foxglove. Its narrow therapeutic range requires TDM.
Which vitamin, after hydroxylation in liver and kidney, stimulates intestinal calcium absorption?
Cholecalciferol becomes 1,25-dihydroxyvitamin D, which induces calcium-binding transport proteins in enterocytes. Vitamin K carboxylates bone and clotting proteins but does not drive absorption.
A man living mainly on maize develops a photosensitive skin rash, diarrhoea and confusion. Deficiency of which vitamin is most likely?
Dermatitis, diarrhoea and dementia define pellagra, caused by niacin deficiency; maize niacin is poorly available and low in tryptophan. Thiamine deficiency causes beriberi and Wernicke encephalopathy without rash.
Intrinsic factor, required for vitamin B12 absorption in the terminal ileum, is:
Parietal cells secrete intrinsic factor, which binds B12 in the duodenum for ileal uptake; autoantibodies against it cause pernicious anaemia. Haptocorrin binds B12 earlier and is removed by pancreatic proteases.
An adult has bone pain, low serum calcium and phosphate, raised alkaline phosphatase and low 25-hydroxyvitamin D. The likely diagnosis is:
Vitamin D deficiency after growth plates close causes defective mineralisation, i.e. osteomalacia; in children it is rickets. Osteoporosis usually shows normal calcium, phosphate and ALP.
Which of these is the richest natural source of preformed vitamin A (retinol)?
Halibut and cod liver oils contain far more retinol than egg yolk or butter. Carrots provide β-carotene, a provitamin, not preformed vitamin A.
Eggs supply many vitamins but contain virtually none of which water-soluble vitamin?
Eggs provide riboflavin, B12 and some thiamine, but no meaningful vitamin C, which comes mainly from fruit and vegetables.
A blood film shows macro-ovalocytes and hypersegmented neutrophils, with low serum folate. The anaemia is:
Folate deficiency impairs DNA synthesis, giving large red cells with immature nuclei and hypersegmented neutrophils. Microcytic hypochromic anaemia points to iron deficiency.
Bleeding gums and poor wound healing in scurvy reflect defective synthesis of:
Vitamin C is needed by prolyl and lysyl hydroxylases; underhydroxylated collagen forms weak triple helices and fragile vessels. Prothrombin depends on vitamin K.
Which vitamin deficiency presents mainly with night blindness and xerophthalmia rather than neurological disease?
Vitamin A deficiency affects the retina and epithelia. B12 deficiency causes subacute combined degeneration, B6 deficiency neuropathy and seizures, and thiamine deficiency dry beriberi and Wernicke encephalopathy.
Chemically, folic acid is:
Folic acid consists of a pteridine ring, p-aminobenzoic acid and glutamate. Pantoyl-β-alanine is pantothenic acid, cyanocobalamin is vitamin B12, and nicotinic acid is niacin.
Which form of vitamin D is the hormonally active metabolite?
Renal 1α-hydroxylase produces calcitriol, which acts on the vitamin D receptor. Calcidiol is the main circulating form and the best test of vitamin D status, but is not active itself.
Raised plasma homocysteine can result from deficiency of several vitamins. Which vitamin deficiency does NOT raise it?
Folate and B12 are needed to remethylate homocysteine, and B6 is needed for transsulfuration. Vitamin C plays no part in homocysteine metabolism.
A malnourished man with chronic alcohol use has confusion, ataxia and ophthalmoplegia. Which vitamin should be replaced urgently?
This triad is Wernicke encephalopathy, caused by thiamine deficiency. Niacin deficiency causes pellagra (dermatitis, diarrhoea, dementia).
Eating large amounts of raw egg white can cause a vitamin deficiency because avidin binds the vitamin tightly. Which vitamin is it?
Avidin binds biotin strongly and stops it being absorbed. Cooking denatures avidin. The same affinity is used in streptavidin–biotin laboratory assays.