A patient presents with cherry-pink mucous membranes, headache, confusion, and a SpO2 reading of 99% on pulse oximetry despite severe tissue hypoxia. The substance responsible is:
- A Carbon monoxide ✓
- B Cyanide
- C Hydrogen sulphide
- D Methanol
Explanation
Carbon monoxide (CO) binds haemoglobin with an affinity approximately 250 times greater than oxygen, forming carboxyhaemoglobin (COHb), which is cherry-red. Standard pulse oximetry cannot distinguish oxyhaemoglobin from COHb, so it falsely reads near-normal saturation despite profound cellular hypoxia. This classic 'normal SpO2 with severe hypoxia' pattern in a patient with cherry-pink skin strongly indicates CO poisoning. Diagnosis requires co-oximetry. Cyanide does not produce a false SpO2 reading, and methanol causes metabolic acidosis with visual disturbances rather than this presentation.
Reference: The Essentials of Forensic Medicine and Toxicology (Narayan Reddy), 34th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.