Which mechanism best explains why neostigmine is preferred over physostigmine for reversing non-depolarizing neuromuscular blockade?

• A Neostigmine has higher lipid solubility and crosses the neuromuscular junction faster
• B Neostigmine directly activates nicotinic receptors at the motor end plate
• C Neostigmine irreversibly inhibits plasma cholinesterase, prolonging its action
• D Neostigmine's quaternary nitrogen structure confines it to the periphery, avoiding central cholinergic side effects ✓

Explanation

Neostigmine is a quaternary ammonium compound that does not cross the blood-brain barrier, so its cholinesterase inhibition is restricted to peripheral synapses. Physostigmine is a tertiary amine that crosses the BBB and can cause central adverse effects (confusion, seizures). Neither drug directly activates nicotinic receptors, and neostigmine reversibly (not irreversibly) inhibits cholinesterase.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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