Suxamethonium Phase II Block — FRCA Primary MCQ
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Correct answer: C — Phase II block occurs after repeated or prolonged administration
Explanation lettering: E = shown as A · A = shown as B · D = shown as C · B = shown as D · C = shown as E
D is correct. Suxamethonium is a depolarising neuromuscular blocking drug. With prolonged exposure or repeated administration producing a large cumulative dose, the initial phase I depolarising block may evolve into a phase II block, which demonstrates features resembling non-depolarising blockade, including train-of-four fade. Suxamethonium is rapidly hydrolysed by plasma cholinesterase rather than hepatic cytochrome P450 enzymes, so A is false. Its usual duration after an intubating dose is approximately 2–6 minutes, not 30–45 minutes, excluding C. It is not a non-depolarising agent, excluding B. Depolarisation causes potassium to leave skeletal muscle, producing a transient increase in serum potassium rather than hypokalaemia; this rise can become dangerous in susceptible pathological states, excluding E.
Reference: electronic Medicines Compendium. Suxamethonium Chloride 50 mg/ml Solution for Injection/Infusion, Summary of Product Characteristics, sections 4.2, 4.4 and 5.1. Updated 2025. https://www.medicines.org.uk/emc/product/3401/smpc