A 4-week-old male infant presents with projectile, non-bilious vomiting after every feeding for the past 3 days. He appears hungry after vomiting. Physical examination reveals a palpable olive-shaped mass in the epigastrium. What is the most likely electrolyte abnormality?

The correct answer to the question "A 4-week-old male infant presents with projectile, non-bilious vomiting after every feeding for the past 3 days. He appears hungry after vomiting. Physical examination reveals a palpable olive-shaped mass in the epigastrium. What is the most likely electrolyte abnormality?" is Option C: Hypochloremic metabolic alkalosis.

The other options — A) Hyperkalemia, B) Hypernatremia, D) Respiratory acidosis — are incorrect for this USMLE Exam Prep question. As explained above: The classic presentation of projectile, non-bilious vomiting in a 4-week-old infant with a palpable epigastric mass is consistent with pyloric stenosis. The persistent loss of gastric acid (HCl) lead…