A 5-Week-Old Boy With Progressive Nonbilious Vomiting: Osmosis USMLE Study Question

June 23, 2017

Answer: C. Hypochloremic hypokalemic metabolic alkalosis

Infantile hypertrophic pyloric stenosis classically presents with progressive nonbilious projectile emesis and an enlarged pylorus (described as an "olive") upon abdominal examination. Prolonged vomiting and a resultant loss of gastric hydrochloric acid (HCI) occur in hypochloremia and metabolic alkalosis. In an attempt to maintain acid-base balance, the cellular hydrogen/potassium pump releases hydrogen into the extracellular space. This outflux of hydrogen causes a reverse influx of potassium, which reduces extracellular potassium levels and causes hypokalemia.

The next step in management is rehydration with intravenous fluids, which often corrects the electrolyte disturbance. The initial diagnostic test of choice is abdominal ultrasonography, which allows the measurement of the length and thickness of the wall muscle. After correction of electrolyte abnormalities, the standard treatment is surgical pyloromyotomy.

Major Takeaway: Infantile hypertrophic pyloric stenosis presents with progressive nonbilious projectile emesis and a palpable pylorus (described as an "olive") upon physical examination. Prolonged vomiting and the resultant loss of gastric HCl produces hypochloremic, hypokalemic metabolic alkalosis.

For more on infantile hypertrophic pyloric stenosis, read here.

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