An Infant brought to the clinic with non-biliousf projectile vomiting. The investigation of choice includes\

(A) Barium meal # Clinical features of Hypertrophic pyloric stenosis:> Characteristically it is a first-born male child that is most commonly affected. The condition is most commonly seen at 4 weeks after birth ranging from the third week to, on rare occasions, the seventh.> Inexplicably, it is the time following birth that seems important and not the child's gestational age.> A premature infant will also develop the condition at about 4 weeks after birth.> Vomiting is the presenting symptom that after 2--3 days becomes forcible and projectile. The child vomits milk and no bile is present. Immediately after vomiting the baby is usually hungry. Weight loss is a striking feature and rapidly the infant becomes emaciated and dehydrated.> Diagnosis can usually be made with a test feed. This may produce characteristic peristaltic waves that can be seen to pass across the upper abdomen.> At the same time, using a warm hand, the abdomen is palpated to detect the lump. Ultrasound examination of abdomen reveals dome shaped, thickened pyloric sphincter.