MLT GlobeChemistry: TDM, toxicology & vitamins – page 5
109 Chemistry MCQs on TDM, toxicology & vitamins with answers and explanations.
To use the acetaminophen (paracetamol) treatment nomogram after a single acute overdose, blood should be drawn at least:
Absorption is incomplete before 4 hours, so earlier levels can underestimate risk. The nomogram starts at 4 hours after ingestion.
In the enzymatic ethanol assay, alcohol dehydrogenase converts ethanol to acetaldehyde. What is measured?
ADH oxidizes ethanol while NAD+ is reduced to NADH. The rise in absorbance at 340 nm is proportional to the ethanol concentration.
A patient has a high anion gap acidosis, a raised osmolal gap and many calcium oxalate crystals in the urine. The most likely poison is:
Ethylene glycol is metabolized to oxalic acid, which forms calcium oxalate crystals and can damage the kidneys. Methanol causes visual damage but not oxalate crystals.
The preferred test to confirm lead exposure in a child is:
Lead is carried mainly in red cells, so whole blood lead is the standard test. Capillary screening results that are raised should be confirmed on a venous whole blood sample.
A patient pulled from a house fire has a normal pulse oximetry reading but headache and confusion. Which test is needed?
Standard pulse oximeters read carboxyhemoglobin as oxyhemoglobin and give falsely normal values. Co-oximetry measures COHb directly at several wavelengths.
An adult with aspirin overdose typically shows which acid-base pattern early on?
Salicylate stimulates the respiratory centre (respiratory alkalosis) and also causes an anion gap metabolic acidosis. The mixed picture is typical of adult salicylate poisoning.
A farm worker has miosis, sweating and excess salivation after pesticide exposure. Which lab result supports organophosphate poisoning?
Organophosphates inhibit acetylcholinesterase; low RBC cholinesterase (and serum pseudocholinesterase) activity supports the diagnosis.
A phenytoin dose is raised by 10%, but the serum level rises by 60% and toxicity develops. This is because phenytoin:
Phenytoin metabolism saturates near therapeutic levels (Michaelis–Menten kinetics), so small dose increases cause disproportionately large rises in level. First-order drugs change level in proportion to dose.
A lab finds some therapeutic drug levels are lower in gel separator tubes than in plain red-top tubes held for the same time. The best explanation is:
Some lipophilic drugs (e.g. certain anticonvulsants and tricyclic antidepressants) can be absorbed into the gel, especially with long contact. Many labs therefore use plain tubes for TDM.
A 500 mg IV dose of a drug is given. Its volume of distribution is 50 L. Assuming instant distribution, what is the expected initial plasma concentration?
Concentration = dose ÷ volume of distribution = 500 mg ÷ 50 L = 10 mg/L. Dividing 50 by 500 or mixing units gives the wrong options.
In the Trinder method for salicylate, what forms the colored product measured?
Ferric ions (Fe3+) in acid react with salicylate to form a violet complex read near 540 nm. Picric acid is used for creatinine (Jaffe), not salicylate.
A man drank homemade spirits and now has blurred vision, high anion gap metabolic acidosis and a raised osmolal gap. Which toxic metabolite causes the eye damage?
Methanol is converted by alcohol dehydrogenase to formaldehyde and then formic acid, which damages the optic nerve and causes acidosis. Oxalic acid comes from ethylene glycol.
A fire victim has severe lactic acidosis and high venous oxygen saturation despite normal carboxyhemoglobin. Which poison is most likely?
Cyanide blocks cytochrome c oxidase, so cells cannot use oxygen; venous blood stays highly oxygenated and anaerobic metabolism produces lactate. Burning plastics can release cyanide.
A workplace urine screen is positive for opiates. Confirmation shows morphine only, at a low level, and the person reports eating poppy seed bread. The correct interpretation is:
Poppy seeds contain morphine and codeine and can give a positive opiate result. Heroin use is proven by its specific metabolite 6-acetylmorphine, which is not produced by poppy seeds.
A workplace urine specimen has creatinine 15 mg/dL (1.3 mmol/L) and specific gravity 1.002. Under common US federal validity criteria, it is classified as:
Creatinine ≥2 but <20 mg/dL with specific gravity >1.0010 but <1.0030 meets the dilute criteria. Substituted requires creatinine below 2 mg/dL with abnormal specific gravity.
Why is red cell folate a better indicator of long-term folate status than serum folate?
Folate enters red cells only during erythropoiesis, so RBC folate reflects the previous 2–3 months. Serum folate changes quickly with recent diet. B12 deficiency can actually lower RBC folate.
A blood ethanol result is 80 mg/dL. Given ethanol molecular weight is 46, what is the result in mmol/L?
80 mg/dL = 800 mg/L; 800 ÷ 46 = 17.4 mmol/L. Forgetting to convert dL to L gives 1.7 mmol/L.
A vancomycin 'trough' level was drawn through the same IV line immediately after the dose was infused. The result will most likely be:
Drawing from the infusion line right after a dose contaminates the sample with drug and misses the true trough time, giving a falsely high level. Troughs are drawn just before the next dose from a different site.
A patient's antiepileptic drug level is repeatedly below the therapeutic range despite a correct prescribed dose. What is the most common cause?
Poor adherence is the most frequent reason for unexpectedly low drug levels. Kidney failure usually raises levels, and a peak sample would give a higher, not lower, result.
A toddler has swallowed many of his mother's iron tablets. Which laboratory test is most useful to assess the severity of poisoning?
Serum iron measured about 4–6 hours after ingestion is used to judge the severity of acute iron poisoning. Ferritin reflects stores and rises slowly.