Which feature most reliably distinguishes thallium poisoning from arsenic poisoning when both can cause painful peripheral neuropathy and Mees' lines?
- A Painful stocking-glove neuropathy
- B Transverse white bands across the fingernails
- C Loss of scalp and body hair with relative sparing of the inner third of the eyebrows ✓
- D Rain-drop pigmentation of the skin
Correct answer: C. Loss of scalp and body hair with relative sparing of the inner third of the eyebrows
Explanation
Thallium characteristically causes diffuse alopecia of scalp, beard, and outer eyebrows while sparing the inner third of the eyebrows, a sign highly suggestive of thallium. Painful neuropathy and Mees' lines occur in both metals, so options A and B do not discriminate. Rain-drop pigmentation is specific to chronic arsenic exposure and actually favours arsenic, making D the fact that kills the closest competing answer.
Reference: Casarett and Doull's Toxicology: The Basic Science of Poisons, 9th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.