A newborn baby not passed meconium for 48 hours since bih, presents with vomiting and distension of abdomen. The most appropriate investigation for evaluation would be?

A i.e., Barium enema study Main indications for water soluble contrast barium enema are neonatal low gastrointestinal obstructionQ, suspected post -necrotizing enterocolitis strictures, Hirschsprung's diseaseQ and after colonic surgery. Colonoscopy has replaced barium enema in inflammatory bowel disease allowing concurrent biopsy & avoiding ionizing radiation. For anorectal malformation, infant is held upside down for 3 - 4 minutes , with a metallic object or coin strapped to the site of anus or metal bougie inseed into the blind anus and a radiograph is taken in inveed position. The distance between the top end of rectal gas and metal indicator indicates the length of malformed rectum. Sufficient gas may have collected in large intestine, 6 hours after bih, to cast radiographic shadow. But sometimes 24 hours wait is required. Neonates with delayed passage of meconium beyond first 24 hours of life with abdominal distension & bilious vomiting are indicative of Hirschsprung's disease. However, it can also present with chronic constipation staing from 1st few weeks of life without fecal soiling in children & adults, in whom per-rectal examination reflects contracted rectal wall and may provide temporary relief from constipation.