Salbutamol (structure below, pKa 9.3):

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BlackTom AI · Accepted Answer

To tackle this question, start by recalling the basic properties of salbutamol and how pKa relates to ionization at physiological pH.
Option 1: 'penetrates membranes less readily when in the ionized form.' Salbutamol has a pKa around 9.3, so at physiological pH (~7.4) it will be largely protonated (ionized). Ionized (charged) molecules cross lipid membranes much less readily than neutral ones, so this statement correctly describes its reduced membrane permeability when ionized.
Option 2: 'is predominantly in the neutral form at physiological pH.' Given a pKa of 9.3, the drug will mostly exist in the ionized form at pH 7.4, not predominantly neutral. This makes the statement false.
Option 3: 'has a selective action on heart muscle.' Salbutamol is known primarily as a selective β2-adrenoceptor agonist with bronchodilatory effects; it has relatively less activity on β1 receptors in the heart. While no drug is perfectly selective, the claim of a selective action on heart muscle is inaccurate for salbutamol and contradicts its clinical use as a bronchodilator with bronchial smooth muscle β2 selectivity.
Option 4: 'is a selective β2-adrenoceptor antagonist.' This is incorrect because salbutamol is a β2-adrenoceptor agonist, not an antagonist. Antagonists block receptors, whereas salbutamol activates β2 receptors to cause bronchodilation.
In summary, the statement about ionization and membrane permeability aligns with the chemistry (pKa 9.3 leading to ionized form at physiological pH reducing membrane crossing), while the other options mischaracterize salbutamol’s ionization state at physiological pH or its pharmacological activity (agonist vs antagonist and receptor selectivity).

PCOL2021 2025 Practice MCQ Section A - 2025

Salbutamol (structure below, pKa 9.3):

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