A laboratory worker who swallowed phenol (carbolic acid) is brought to hospital with white eschars on the lips that have turned brownish. On standing overnight, his urine sample develops a striking colour change. What change is expected?

• A Port-wine colour due to porphyrins
• B Bright red colour due to free haemoglobin
• C Milky appearance due to fat globules
• D Greenish-black smoky discoloration due to oxidised metabolites ✓

Explanation

Phenol poisoning produces carboluria: the urine turns smoky greenish-black on exposure to air because phenol is oxidised to hydroquinone and pyrocatechol, which darken on standing. This is a classic forensic marker. Port-wine urine indicates porphyria, chyluria is milky from lymph in the urine, and frank haemoglobinuria occurs in intravascular haemolysis such as in copper sulphate or arsine poisoning, none of which follows phenol ingestion.

Reference: Essentials of Forensic Medicine and Toxicology, R.K. Narayan Reddy, 33rd ed.

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Written and medically reviewed by the StethoPrep medical team.