Hypertrophic pyloric stenosis
USMLE Step 2 CKDifficulty: easyLast updated:
A 4-week-old firstborn boy is brought to the clinic because of forceful, nonbilious vomiting immediately after feeds for 5 days. He seems hungry and feeds eagerly after vomiting. He has had fewer wet diapers and has lost weight. Examination shows dry mucous membranes; a firm, mobile, olive-sized mass is palpable in the epigastrium.
Which of the following sets of serum findings is most likely?
Answer and explanation
Correct answer: D. Chloride 88 mEq/L, potassium 3.0 mEq/L, bicarbonate 34 mEq/L
Projectile nonbilious vomiting after feeds in a hungry 3- to 6-week-old infant with an olive-like epigastric mass is typical of hypertrophic pyloric stenosis. Repeated vomiting of gastric contents loses hydrochloric acid, causing hypochloremia and metabolic alkalosis. Volume depletion then activates aldosterone, which increases renal potassium loss. The source states that as the illness progresses, infants fail to gain weight, become malnourished, and develop dehydration with hypochloremic, hypokalemic metabolic alkalosis. Diagnosis is by abdominal ultrasound showing a thickened (typically ≥ 3 mm) and elongated pylorus. Treatment is pyloromyotomy, after the dehydration and electrolyte abnormalities are corrected.
Each option explained
- A. Chloride 110 mEq/L, potassium 3.0 mEq/L, bicarbonate 15 mEq/LIncorrect. Hyperchloremic acidosis with hypokalemia suggests diarrheal bicarbonate loss or renal tubular acidosis.
- B. Chloride 103 mEq/L, potassium 4.4 mEq/L, bicarbonate 24 mEq/LIncorrect. Normal electrolytes would be unexpected after 5 days of vomiting with dehydration.
- C. Chloride 100 mEq/L, potassium 5.9 mEq/L, bicarbonate 18 mEq/LIncorrect. Acidosis with hyperkalemia suggests adrenal insufficiency (e.g., congenital adrenal hyperplasia), not pyloric obstruction.
- D. Chloride 88 mEq/L, potassium 3.0 mEq/L, bicarbonate 34 mEq/LCorrect. Low chloride, low potassium, and high bicarbonate describe hypochloremic, hypokalemic metabolic alkalosis.
- E. Chloride 112 mEq/L, potassium 5.8 mEq/L, bicarbonate 14 mEq/LIncorrect. Hyperchloremic metabolic acidosis with hyperkalemia does not result from loss of gastric acid.
Key point
Pyloric stenosis causes hypochloremic, hypokalemic metabolic alkalosis; confirm with ultrasound and correct electrolytes before pyloromyotomy.
From the source
“With progression of illness, children do not gain weight, become malnourished, and develop dehydration with electrolyte and acid-base abnormalities (hypochloremic, hypokalemic metabolic alkalosis).”