A 20-year-old woman presents 4 hours after a deliberate salicylate overdose. She is febrile, hyperventilating, and her arterial pH shows mixed acid-base disturbance. Along with fluid replacement, sodium bicarbonate infusion is administered. The pharmacokinetic basis for this intervention is:

• A Alkalinizing the plasma shifts salicylate out of tissues into the vascular compartment
• B Alkalosis converts salicylate to its unionized form, promoting passive reabsorption
• C Bicarbonate saturates the glycine conjugation pathway, forcing faster renal clearance
• D Alkalinizing the urine ionizes salicylate, trapping it in the tubular lumen and increasing its excretion ✓

Explanation

Salicylic acid is a weak acid. Raising urine pH with sodium bicarbonate increases the fraction of salicylate present as the ionized lipophobic form inside the tubular lumen. Ionized molecules cannot diffuse back across tubular epithelium, so they remain trapped and are excreted in urine, a process called ion trapping. Option A confuses this with redistribution, and option B states exactly the reverse of the intended effect.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

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