A 3-week-old boy with projectile vomiting has serum sodium 132 mEq/L, potassium 2.8 mEq/L, chloride 84 mEq/L, pH 7.52, and bicarbonate 34 mEq/L. The acid-base disturbance arises because:
- A Volume contraction activates the renin-angiotensin system, promoting hydrogen secretion and bicarbonate reabsorption despite alkalemia
- B Gastric juice is rich in hydrochloric acid, so its loss leaves unbuffered bicarbonate in the plasma ✓
- C Hypokalemia drives potassium out of cells in exchange for hydrogen ions entering cells
- D The infant hyperventilates to compensate for the alkalosis, generating additional bicarbonate
Correct answer: B. Gastric juice is rich in hydrochloric acid, so its loss leaves unbuffered bicarbonate in the plasma
Explanation
Pyloric stenosis causes loss of gastric hydrochloric acid without pancreatic bicarbonate loss. Removing hydrogen and chloride leaves excess bicarbonate, producing hypochloremic hypokalemic metabolic alkalosis. Volume contraction does sustain the alkalosis via aldosterone-driven hydrogen secretion, but the initiating mechanism is direct acid loss, which is what the question asks. Option C describes intracellular shifts seen in other settings and option D reverses the actual respiratory compensation.
Reference: Nelson Textbook of Pediatrics, 22nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.