MLT GlobeForensic Science: Forensic toxicology – page 2
45 Forensic Science MCQs on Forensic toxicology with answers and explanations.
After a single use of cocaine, its main urinary metabolite benzoylecgonine is typically detectable for about:
Cocaine itself has a short half-life, but benzoylecgonine is excreted longer and is generally detectable in urine for roughly 2-4 days after occasional use, longer after heavy use.
In a chronic heavy cannabis user, urinary THC-COOH may remain detectable after cessation for up to about:
THC is highly lipophilic and stored in adipose tissue, releasing slowly. In chronic heavy users the metabolite THC-COOH may be detectable in urine for weeks, whereas occasional users usually test negative within days.
Postmortem redistribution refers to:
After death, drugs diffuse from organs with high concentrations, such as lungs, liver and myocardium, into adjacent blood. Central blood concentrations may therefore rise well above those present at death.
Sodium fluoride is added to blood intended for cocaine analysis mainly to:
Cocaine continues to be hydrolysed in drawn blood by cholinesterases and chemically. Fluoride inhibits enzymatic breakdown, and refrigeration further improves stability, preserving the parent drug concentration.
Why is liver tissue often collected for postmortem toxicology?
The liver metabolizes and accumulates many drugs, so concentrations are often high and detection is easier. It is especially useful in decomposed bodies or when blood is scanty, though interpretation is less established.
A key advantage of LC-MS/MS over GC-MS in forensic toxicology is that it:
Liquid chromatography does not require volatilization, so polar, non-volatile and thermally unstable compounds, including glucuronides, can be analysed directly, and tandem MS gives high sensitivity and specificity.
Why is a deuterated analogue of the analyte commonly used as the internal standard in quantitative MS assays?
A deuterated internal standard co-extracts and co-elutes almost identically to the analyte, yet has a different mass. It compensates for losses during extraction and instrument variability, improving accuracy.
When using the Rumack-Matthew nomogram to assess paracetamol overdose risk, the plasma level should be drawn:
Absorption is usually incomplete before 4 hours, so earlier levels may underestimate risk. The nomogram applies only to a single acute ingestion with a known time, not to staggered or chronic overdoses.
For assessing lead poisoning, the recommended specimen is:
About 99% of blood lead is bound to red cells, so whole blood is analysed. Lead-free collection tubes, usually EDTA, are used to avoid contamination; serum levels are far too low for assessment.
A fatal overdose involves fentanyl, but the routine opiate immunoassay is negative. The most likely reason is that fentanyl:
Opiate immunoassays target morphine-like structures. Fentanyl is a synthetic phenylpiperidine opioid that does not cross-react, so specific fentanyl immunoassays or mass spectrometry are needed.
A patient taking clonazepam has a negative urine benzodiazepine immunoassay. The most likely explanation is:
Many benzodiazepine immunoassays are calibrated to oxazepam or nordiazepam and detect some metabolites poorly, including 7-aminoclonazepam and lorazepam glucuronide. Low doses may be missed without hydrolysis or MS methods.
Adding sodium dithionite in alkaline conditions to urine produces a blue colour in poisoning with:
Sodium dithionite reduces paraquat to a blue free radical. The colour intensity roughly reflects concentration, making it a quick bedside or laboratory screening test for this herbicide.
Aluminium phosphide pesticide tablets are highly toxic because, on contact with moisture, they release:
Aluminium phosphide reacts with water and gastric acid to release phosphine, which inhibits mitochondrial respiration and causes shock and cardiotoxicity. Silver nitrate paper blackening is a simple screening test.
Why must a postmortem drug concentration listed as 'fatal' in reference tables be interpreted cautiously?
Tolerance, postmortem redistribution, specimen site and co-ingested drugs mean therapeutic, toxic and fatal ranges overlap. Interpretation requires case history, autopsy findings and specimen details, not a single number.
The main purpose of solid-phase extraction in toxicology sample preparation is to:
Solid-phase extraction retains analytes on a sorbent while proteins, salts and other matrix components are washed away. Elution in a small volume concentrates analytes, improving sensitivity and instrument performance.
Why is erythrocyte acetylcholinesterase considered a better indicator of organophosphate toxicity than plasma butyrylcholinesterase?
Red-cell acetylcholinesterase is the same enzyme as at neuronal synapses, so its inhibition reflects synaptic inhibition more closely. Plasma butyrylcholinesterase is more sensitive but varies with liver function, genetics and pregnancy.
Shortly after ingestion, before much metabolism has occurred, which laboratory abnormality is most expected in methanol poisoning?
Unmetabolized methanol is osmotically active, so the osmolal gap rises early. As it is converted to formic acid, the osmolal gap falls while the anion gap and acidosis increase.
A worker's urine total arsenic is high after a seafood dinner. Which approach best clarifies whether the exposure is toxic?
Seafood contains arsenobetaine, a relatively non-toxic organic arsenic excreted unchanged. Speciation (for example HPLC-ICP-MS) separates it from toxic inorganic arsenic and its methylated metabolites.
Which urinary analyte is specific evidence of heroin use rather than morphine or codeine intake?
Heroin is rapidly deacetylated to 6-monoacetylmorphine and then morphine. 6-MAM is not produced from morphine, codeine or poppy seeds, but its short detection window means absence does not exclude heroin.
Which drug property most increases the likelihood of significant postmortem redistribution?
Lipophilic basic drugs with large volumes of distribution, such as tricyclic antidepressants and digoxin, are concentrated in tissues. After death they leach into blood, raising central-to-peripheral concentration ratios.