A forensic toxicologist uses hair analysis for retrospective drug detection. The approximate window of detection for cocaine in a single strand of hair 10 cm long, given a hair growth rate of 1 cm/month, is:

• A 3 months maximum regardless of length, as cocaine metabolites are labile in hair
• B Only the most recent 2 cm (2 months) can be analysed reliably
• C Hair cannot detect cocaine as it is a water-soluble molecule that does not incorporate into the hair shaft
• D 10 months; each 1 cm segment represents approximately one month of drug history ✓

Explanation

Scalp hair grows approximately 1 cm per month. Drugs incorporate into growing hair via diffusion from blood, sweat, and sebum. Segmental hair analysis (cutting hair into 1 cm sections) allows month-by-month reconstruction of drug use history. A 10 cm hair shaft represents approximately 10 months of history. Cocaine and its metabolite benzoylecgonine are highly stable in hair and detectable for months to years. Hair analysis provides a far longer retrospective window than blood (hours) or urine (days to weeks), making it invaluable for chronic exposure assessment.

Reference: The Essentials of Forensic Medicine and Toxicology (Narayan Reddy), 34th ed.

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