MLT GlobeForensic Science: Forensic toxicology
45 Forensic Science MCQs on Forensic toxicology with answers and explanations.
In a postmortem case, which blood specimen is generally preferred for quantitative drug interpretation?
Femoral blood is anatomically distant from drug reservoirs such as the liver, lungs and gut, so it is less affected by postmortem redistribution and diffusion than central or cavity blood.
A urine immunoassay screen for opiates is positive. Why is confirmation by GC-MS or LC-MS/MS required before reporting?
Immunoassays rely on antibody cross-reactivity and can respond to structurally related compounds. Confirmation by chromatography-mass spectrometry, a different analytical principle, specifically identifies and quantifies the analyte, making the result forensically defensible.
Why is vitreous humor a valuable specimen in postmortem toxicology?
The vitreous is enclosed within the eye, relatively protected from bacterial invasion, trauma and diffusion from the gut, so it remains useful when blood is contaminated, decomposed or unavailable.
Carbon monoxide is toxic chiefly because it:
Carbon monoxide binds hemoglobin roughly 200-250 times more avidly than oxygen, forming carboxyhemoglobin, reducing oxygen carriage and shifting the dissociation curve left, causing tissue hypoxia.
Which postmortem lividity colour classically suggests carbon monoxide poisoning?
Carboxyhemoglobin is bright red, giving lividity and tissues a cherry-pink colour. Similar colouring can occur with cyanide or cold exposure, so laboratory confirmation of carboxyhemoglobin is essential.
Organophosphate insecticides produce their toxic effects mainly by:
Organophosphates phosphorylate the active site of acetylcholinesterase, so acetylcholine accumulates at muscarinic, nicotinic and central synapses, producing a cholinergic crisis.
Cyanide causes cellular hypoxia by inhibiting which enzyme?
Cyanide binds ferric iron in cytochrome c oxidase (complex IV), halting mitochondrial oxidative phosphorylation, so cells cannot use oxygen despite adequate delivery, causing lactic acidosis.
A positive urine drug screen for a drug of abuse most accurately indicates that the person:
Urine collects drugs and metabolites over time, often after effects have gone. A positive result shows prior exposure within the detection window but cannot establish impairment, timing of dose or dependence.
What is the main toxicological value of analysing gastric contents at autopsy?
Large amounts of drug or tablet residue in the stomach indicate oral intake and may support recent overdose. Gastric concentration does not reflect systemic exposure, so it complements blood analysis.
In GC-MS, a compound is identified by combining which two pieces of information?
Gas chromatography separates compounds, each eluting at a characteristic retention time. The mass spectrometer then produces a fragmentation pattern compared with reference spectra, giving high-confidence identification.
In paracetamol (acetaminophen) overdose, liver injury is caused by which reactive metabolite?
Excess paracetamol saturates glucuronidation and sulfation, so more is oxidized by CYP2E1 to NAPQI. When glutathione is depleted, NAPQI binds hepatocyte proteins, causing centrilobular necrosis.
Trinder's reagent, which contains ferric ions, gives a violet colour with urine containing:
Ferric ions form a violet complex with the phenolic group of salicylate, providing a rapid, simple qualitative spot test. Positive results should be followed by quantitative serum salicylate measurement.
Which pupillary sign is classically associated with opioid overdose?
Opioids stimulate the Edinger-Westphal nucleus via mu receptors, producing pinpoint pupils, together with respiratory depression and reduced consciousness. Mixed ingestions or hypoxia may alter this sign.
Postmortem toxicology specimens awaiting analysis for several months are best stored:
Freezing, generally around -20 degrees C, slows chemical and microbial degradation of most analytes. Each specimen stays in its own sealed, labelled container to preserve identity and chain of custody.
Segmental analysis of a scalp hair sample is mainly used to obtain which information?
Scalp hair grows about 1 cm per month, and drugs incorporated into the shaft remain there. Cutting hair into segments allows an approximate month-by-month history of exposure, but not acute intake.
Which laboratory method is routinely used to measure carboxyhemoglobin saturation in a blood sample?
CO-oximeters measure absorbance at several wavelengths to distinguish oxyhemoglobin, deoxyhemoglobin, carboxyhemoglobin and methemoglobin. Conventional pulse oximetry cannot distinguish carboxyhemoglobin from oxyhemoglobin.
Which laboratory finding best supports suspected organophosphate exposure in a living patient?
Organophosphates inhibit cholinesterases, so markedly depressed erythrocyte acetylcholinesterase or plasma butyrylcholinesterase activity supports exposure; the parent compound or metabolites can be confirmed by chromatography.
Blood for cyanide analysis should be collected in a well-filled, sealed container and analysed promptly because:
Cyanide is volatile and can be lost from poorly sealed or partly filled containers; it may also be formed in stored or putrefied samples. Prompt analysis of properly sealed specimens gives the most reliable result.
The severe acidosis and visual damage in methanol poisoning are mainly due to which metabolite?
Alcohol dehydrogenase converts methanol to formaldehyde, which is rapidly oxidized to formic acid. Formate inhibits mitochondrial cytochrome oxidase, damaging the optic nerve and causing a high anion gap metabolic acidosis.
For suspected chronic arsenic exposure, which specimens are most useful for documenting long-term intake?
Arsenic binds to keratin sulfhydryl groups, so it accumulates in growing hair and nails and remains detectable for months, providing evidence of chronic exposure long after blood levels fall.