USMLE Step 2 CK High-Yield Practice Test

In infants, nonbilious projectile vomiting is a classic presentation of pyloric stenosis, a condition characterized by hypertrophy of the pyloric muscle, which leads to gastric outlet obstruction. This typically occurs around 3 to 12 weeks of age and is associated with other signs such as dehydration and weight loss. The projectile nature of the emesis suggests that the contents are coming from the stomach and not the intestines, which would introduce bile into the vomit if there were an obstruction downstream of the pylorus.

When a patient is suspected to have pyloric stenosis, the first step in management involves addressing any metabolic abnormalities, particularly electrolyte imbalances that may arise due to the vomiting, such as hypochloremic metabolic alkalosis. This is critical before surgical intervention, as rehydrating and normalizing electrolyte levels help prepare the patient for any subsequent interventions, such as surgical pyloromyotomy, which relieves the obstruction.

In contrast, other conditions listed have different presentations and management approaches; for instance, gastroesophageal reflux may involve regurgitation rather than projectile vomiting, while intestinal obstruction and Hirschsprung's disease usually present with different clinical findings that do not match the classic description of pyloric stenosis. Therefore, recognizing the