Skip to content

Quiz: Forensic Toxicology and Chemical Analysis

Test your understanding of the ADME pharmacokinetic pathway, drug schedules, presumptive testing, GC-MS analysis, and BAC calculations with these questions.

1. The ADME pathway describes four phases of drug processing in the body. In which phase does the liver's cytochrome P450 enzyme system convert parent drugs into metabolites?

  1. Absorption, because the liver processes substances absorbed from the gastrointestinal tract before they enter systemic circulation
  2. Distribution, because the cytochrome P450 system controls how drugs spread through body tissues
  3. Metabolism, because the liver biotransforms the parent compound into more water-soluble metabolites for excretion
  4. Elimination, because cytochrome P450 enzymes actively pump drug molecules into the renal tubules for urinary excretion
Show Answer

The correct answer is C. Metabolism (biotransformation) is the phase in which the liver — using the cytochrome P450 (CYP) enzyme system — converts parent drug compounds into metabolites. These metabolites are typically more water-soluble, facilitating excretion. Forensic toxicologists often test for metabolites rather than parent drugs because metabolites persist longer in urine and specific metabolites are diagnostic of specific drugs (e.g., benzoylecgonine is unique to cocaine use). First-pass hepatic metabolism in oral absorption reduces the active dose before systemic distribution.

Concept Tested: Drug Metabolism

2. THC (the active compound in cannabis) can be detected in urine for weeks after use, while more water-soluble drugs clear within days. Which pharmacokinetic property explains this extended detection window for THC?

  1. THC has an extremely long half-life because it is poorly metabolized by the cytochrome P450 system
  2. THC is highly lipid-soluble and accumulates in fatty tissue, releasing slowly back into the bloodstream over time
  3. THC binds strongly to plasma proteins, which prevents it from being filtered by the kidneys
  4. THC is reabsorbed from the urine in the renal tubules, recycling it back into the bloodstream repeatedly
Show Answer

The correct answer is B. THC is highly lipid-soluble, meaning it readily crosses cell membranes and accumulates in fatty tissue during the distribution phase. After the peak blood concentration declines, THC slowly releases back from fat stores into the bloodstream over days to weeks, maintaining detectable urine metabolite levels far longer than water-soluble drugs that clear rapidly through renal filtration. This is why urine drug tests for cannabis can remain positive for weeks in heavy users, while many other drugs clear in 2–4 days.

Concept Tested: Drug Distribution

3. Under the Controlled Substances Act, which schedule contains substances with the highest abuse potential, no currently accepted medical use, and the strictest restrictions?

  1. Schedule V, because the "V" designation indicates the highest severity level in federal law
  2. Schedule II, because opioids and stimulants with the most common abuse are classified here
  3. Schedule IV, because benzodiazepines — the most widely abused prescription drugs — are classified here
  4. Schedule I, because substances in this category have no accepted medical use and the highest abuse potential
Show Answer

The correct answer is D. Schedule I substances have the highest abuse potential, no currently accepted medical use in the United States, and cannot be legally prescribed. Examples include heroin, LSD, MDMA, psilocybin, and cannabis under federal law. Schedule II substances (opioids, methamphetamine, cocaine as a topical anesthetic) also have high abuse potential but have recognized medical applications with restrictions. Schedule I is the most restrictive category — these substances can only be used in federally authorized research.

Concept Tested: Drug Schedule Classifications

4. The Scott reagent test is used to presumptively identify which specific drug, and what makes the two-step procedure more specific than a one-step color test?

  1. Cannabis — the two-step procedure distinguishes THC from other cannabinoids present in hemp products
  2. Methamphetamine — the second step distinguishes methamphetamine from amphetamine using acid hydrolysis
  3. Cocaine — other compounds that initially produce a blue color turn pink or clear after hydrochloric acid addition, while cocaine remains blue
  4. Heroin — the second step distinguishes morphine-based opioids from synthetic opioids using oxidative reagents
Show Answer

The correct answer is C. The Scott reagent test (cobalt thiocyanate + hydrochloric acid) is specific for cocaine. The two-step procedure improves specificity: in step one, cocaine (and some other compounds) produce a blue color with cobalt thiocyanate. In step two, hydrochloric acid is added — cocaine's blue color persists, but other compounds that initially appeared blue turn pink or clear. This sequential confirmation reduces false positives compared to single-step color tests. All presumptive positives still require GC-MS confirmation before court admission.

Concept Tested: Scott Reagent Test

5. In GC-MS analysis, the gas chromatography step separates compounds by their retention times, but the mass spectrometry step provides definitive identification. Why can GC alone not definitively identify an unknown compound?

  1. GC is not sensitive enough to detect drug compounds at the concentrations found in forensic samples
  2. Different compounds can share similar retention times; only the mass spectrum provides a unique molecular fingerprint for each compound
  3. GC cannot separate drug metabolites from parent compounds because they have identical molecular weights
  4. The GC detector is non-specific and produces the same signal for all organic compounds regardless of structure
Show Answer

The correct answer is B. Gas chromatography separates compounds by their interaction with the column's stationary phase, producing characteristic retention times. However, different compounds — especially structurally similar ones — can have the same or very similar retention times, making identity ambiguous from GC alone. Mass spectrometry ionizes each separated compound and generates a fragmentation pattern (mass spectrum) that is unique to the molecule's structure — essentially a molecular fingerprint. The mass spectrum is compared against reference databases (such as the NIST library with 350,000+ spectra) for definitive identification.

Concept Tested: GC-MS Analysis

6. A 90 kg male (Widmark factor r = 0.68) consumed 56 grams of alcohol. Using the Widmark formula (BAC = A ÷ (W × r)), what is his estimated BAC?

  1. Approximately 0.091% — above the legal driving limit of 0.08%
  2. Approximately 0.056% — below the legal driving limit
  3. Approximately 0.14% — double the legal driving limit
  4. Approximately 0.038% — well below the legal driving limit
Show Answer

The correct answer is A. Applying the Widmark formula: BAC = 56 ÷ (90,000 × 0.68) = 56 ÷ 61,200 = 0.000915 g/g. Converting to percentage (g/dL): approximately 0.091%. This exceeds the legal driving limit of 0.08% in most U.S. states. Note that the Widmark formula expresses BAC as a percentage relative to blood volume, and 56 grams is equivalent to 4 standard drinks (each 14 grams), consistent with exceeding the legal limit for a 90 kg male consuming 4 drinks over a short period.

Concept Tested: Blood Alcohol Concentration

7. BAC retro-extrapolation is used to estimate a driver's BAC at the time of a traffic stop rather than at the later time of blood collection. Using an elimination rate of 0.017 g/dL per hour, what was the estimated BAC 2 hours before a measured BAC of 0.05%?

  1. 0.016% — the BAC was below the legal limit at the earlier time because elimination was rapid
  2. 0.084% — because 2 hours × 0.017 g/dL/hr = 0.034, added to 0.05% gives 0.084%
  3. 0.033% — because 0.05% minus 0.017% per hour equals the original BAC
  4. 0.05% — because BAC retro-extrapolation cannot be applied without knowing the exact body weight
Show Answer

The correct answer is B. Retro-extrapolation formula: BAC at event = BAC measured + (hours elapsed × elimination rate) = 0.05 + (2 × 0.017) = 0.05 + 0.034 = 0.084%. The BAC was 0.084% at the time of the event — above the 0.08% legal limit — even though the measured BAC taken 2 hours later was only 0.05%. This demonstrates why retro-extrapolation matters legally: a below-limit blood draw result does not necessarily mean the driver was below the limit at the time of the traffic stop.

Concept Tested: BAC Retro-Extrapolation

8. Cyanide poisoning and carbon monoxide poisoning both cause death by depriving cells of oxygen. What is the key difference in their mechanisms?

  1. Cyanide destroys red blood cells; carbon monoxide prevents oxygen from binding to hemoglobin in the lungs
  2. Cyanide blocks the mitochondrial enzyme cytochrome c oxidase, preventing cells from using oxygen; carbon monoxide binds hemoglobin with 240× greater affinity than oxygen, preventing oxygen transport
  3. Cyanide causes histotoxic hypoxia only in the brain; carbon monoxide affects all tissues equally
  4. Cyanide acts through the respiratory system; carbon monoxide acts through the cardiovascular system
Show Answer

The correct answer is B. Cyanide causes histotoxic hypoxia — it blocks cytochrome c oxidase in mitochondria, preventing cells from using oxygen even when oxygen is plentiful (blood oxygen levels are normal). Carbon monoxide causes hypemic hypoxia — it binds hemoglobin with approximately 240 times greater affinity than oxygen, displacing oxygen from red blood cells and preventing its transport to tissues. Both produce cherry-red tissue/blood coloration, but via different mechanisms: cyanide leaves oxygen in the blood unused; CO fills the hemoglobin with CO instead of oxygen.

Concept Tested: Cyanide Poisoning

9. Heavy metal exposure can be analyzed in hair samples to reconstruct a timeline of exposure. What physical property of hair makes this timeline reconstruction possible?

  1. Hair changes color in response to heavy metal exposure, with different metals producing different color bands
  2. Hair grows at approximately 1 cm per month, so metal concentrations along the shaft provide a month-by-month exposure history as heavy metals incorporate into the growing keratin matrix
  3. Heavy metals bind to melanin granules in the hair cortex, making them visible under polarized light microscopy
  4. Hair retains heavy metals in its cuticle scales, which are shed and replaced at a known rate tied to the exposure period
Show Answer

The correct answer is B. Hair grows at approximately 1 cm per month, and heavy metals — arsenic, lead, mercury, thallium — incorporate into the keratin matrix of the hair shaft as it grows. Because the metal is locked into the hair structure, the concentration at any given point along the shaft reflects the blood metal level at the time that portion grew. Analyzing metal concentrations along segments of the hair shaft provides a month-by-month record of heavy metal exposure history, which can be valuable in poisoning investigations where exposure occurred over extended time periods.

Concept Tested: Heavy Metal Toxicology

10. The Duquenois-Levine test is used as a presumptive field test for cannabis. What happens at the third step of the test that confirms a positive result?

  1. A blue color develops that persists after addition of hydrochloric acid, distinguishing cannabis from other plant materials
  2. Chloroform is added — the chloroform layer turns purple while the aqueous layer clears, a distinctive three-stage positive for cannabis
  3. The sample produces an orange fluorescence under UV light, indicating cannabinoid compounds are present
  4. Addition of silver nitrate produces a white crystalline precipitate characteristic of cannabis alkaloids
Show Answer

The correct answer is B. The Duquenois-Levine test is a three-step sequential procedure: (1) Duquenois reagent + cannabis material → purple color; (2) addition of hydrochloric acid — purple persists; (3) addition of chloroform — the chloroform (organic) layer turns purple while the aqueous layer clears. This partition of the purple color into the organic chloroform layer is the diagnostic step. The test targets terpenoid and cannabinoid compounds specific to cannabis. Like all presumptive tests, a positive result requires GC-MS confirmation for court admission.

Concept Tested: Duquenois-Levine Test