A patient on phenelzine (an MAOI) for depression eats aged cheese and develops a severe hypertensive crisis. The correct explanation is:
- A Phenelzine blocks adrenergic receptors, sensitising vessels to dietary amines
- B Tyramine directly activates MAO-B in sympathetic nerve terminals
- C Phenelzine inhibits MAO-A, preventing tyramine metabolism; unmetabolised tyramine is absorbed and releases norepinephrine, causing hypertension ✓
- D Phenelzine inhibits COMT, preventing adrenaline breakdown after tyramine ingestion
Correct answer: C. Phenelzine inhibits MAO-A, preventing tyramine metabolism; unmetabolised tyramine is absorbed and releases norepinephrine, causing hypertension
Explanation
Non-selective MAO inhibitors (e.g., phenelzine, tranylcypromine) block both MAO-A and MAO-B in gut and liver, preventing first-pass metabolism of dietary tyramine. Absorbed tyramine enters sympathetic terminals where it displaces and releases stored norepinephrine, causing a potentially fatal adrenergic surge with hypertension, headache, and stroke risk — the 'cheese reaction'. Tyramine-containing foods (aged cheese, wine, yeast extracts) must be strictly avoided.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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Written and medically reviewed by the StethoPrep medical team.