A 4-week-old boy presents with a 10-day history of vomiting that has increased in frequency and forcefulness. The vomitus is not bile stained. The child feeds avidly and looks well, but he has been losing weight. An ultrasound of the abdomen is shown. Which of the following should be the next step in this patient's management?

(a) Source: (Hay et al, p 600. Kliegman et al, pp 1274-1275. McMillan et al, pp 371-372. Rudolph et al, pp 1420-1421.) A history of nonbilious vomiting for 10 days in a child of this age who does not look ill points to infantile hypertrophic pyloric stenosis as the most likely diagnosis; surgical consultation for a likely pyloromyotomy is indicated. The ultrasound in the question demonstrates the thickened pylorus. The incidence of this condition in infants is between 1:200 and 1:750, with males affected more often than females. Although there is no specific pattern of inheritance, a familial incidence has been observed in about 15% of patients. Information about predisposing ancestry is conflicting, as are data concerning the assertion of a firstborn predilection. Metabolic alkalosis with low serum potassium and chloride levels is frequently seen in pyloric stenosis as a result of loss of gastric contents from vomiting. A child with a small-bowel obstruction (who may require an upper GI with small-bowel follow through to help diagnose the point of obstruction) should develop bilious emesis and should not look well 10 days into the illness; similarly, a child with intussusception (who requires an air contrast enema, which is both diagnostic and perhaps therapeutic) would be markedly ill at this point. Gastroenteritis (IV fluids alone to maintain hydration) does not usually last for 10 days. A brain tumor causing increased intracranial pressure could present with isolated emesis, but should show other symptoms such as irritability and somnolence.