A 24-year-old woman is brought to the emergency department after ingesting a large quantity of phenobarbital. She is drowsy but haemodynamically stable. Intravenous sodium bicarbonate is administered along with supportive care. The pharmacokinetic principle underlying urinary alkalinisation in this setting is:

• A Alkalinisation converts phenobarbital to its unionised form, enhancing passive reabsorption
• B Alkalinisation competitively inhibits the renal transporter responsible for phenobarbital reabsorption
• C Alkalinisation traps the weak acid phenobarbital in its ionised form within the tubular lumen, reducing reabsorption ✓
• D Alkalinisation increases glomerular filtration of protein-bound phenobarbital by displacing it from albumin

Explanation

Phenobarbital is a weak acid (pKa about 7.4). Raising urine pH increases the fraction ionised in the tubular lumen according to the Henderson-Hasselbalch relationship. Ionised molecules cannot diffuse back across lipid membranes, so they are trapped in the urine and excreted, a process called ion trapping. Option A reverses the chemistry. Phenobarbital reabsorption is by passive diffusion, not active transport, eliminating option B, and filtration applies only to unbound drug, eliminating option D.

Reference: Goodman and Gilman's The Pharmacological Basis of Therapeutics, 14th ed.

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