MLT GlobeChemistry: TDM, toxicology & vitamins – page 6
109 Chemistry MCQs on TDM, toxicology & vitamins with answers and explanations.
In tyrosine breakdown, p-hydroxyphenylpyruvate dioxygenase converts its substrate to homogentisate. Which vitamin is needed for this enzyme to work?
The dioxygenase needs vitamin C to keep its iron reduced; scurvy can cause transient tyrosinaemia. Niacin is not required.
Which range is closest to the normal serum retinol (vitamin A) concentration in adults?
Adult serum retinol is roughly 30–70 µg/dL (about 1–2.5 µmol/L); values below 20 µg/dL indicate deficiency.
Arsenite poisoning inhibits pyruvate dehydrogenase by binding which cofactor's vicinal thiol groups?
Trivalent arsenite binds the reduced dithiol of lipoamide, blocking PDH and alpha-ketoglutarate dehydrogenase. Arsenate acts differently, by uncoupling substrate-level phosphorylation.
A low serum copper level is most likely in which patient?
Malabsorption reduces copper uptake, causing hypocupremia. Pregnancy, estrogen and inflammation raise ceruloplasmin and so increase serum copper.
To assess chronic arsenic exposure over several months, which specimen is most useful?
Arsenic binds keratin in hair and nails, recording exposure over months. Blood arsenic clears quickly; urine reflects recent exposure.
A patient started on carbamazepine has good levels at week 1, but at week 4 the level is lower on the same dose with good adherence. The most likely reason is:
Carbamazepine induces the CYP3A4 enzymes that metabolize it, so clearance increases over the first few weeks and levels fall. Doses often need adjusting after this autoinduction.
Current consensus guidelines for serious MRSA infections recommend vancomycin dosing be guided by:
The 2020 ASHP/IDSA/SIDP guideline moved from trough-only monitoring to AUC-guided dosing, targeting AUC/MIC 400–600 (MIC 1 mg/L). This balances efficacy and nephrotoxicity better than trough targets.
A patient has a raised osmolal gap, ketones in blood and urine, normal glucose and no metabolic acidosis. Which ingestion is most likely?
Isopropanol is metabolized to acetone, a ketone that is not an acid, so ketosis and a raised osmolal gap occur without acidosis. Methanol and ethylene glycol produce organic acids and a high anion gap.
After digoxin-specific antibody Fab fragments are given for toxicity, total serum digoxin by immunoassay rises greatly. The best explanation is:
Fab fragments pull digoxin from tissue into plasma, where it is tightly bound and inactive. Total immunoassay results become misleading, so free (ultrafiltrate) digoxin is measured if needed.