This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pediatrics
A neonate presented with fever, lethargy, abdominal distension, vomiting and constipation. Clinically he was diagnosed as volvulus neonatarum with suspected perforation. Best investigation would be: 
Select and explain. 
A. Plain X-ray, B. Barium enema, C. Upper GI endoscopy, D. Barium meal follow through
A: Plain X-ray
Answer: A: Plain X-ray 
Explanation: In a case of suspected perforation plain x-ray should be performed. It shows free air under the diaphragm and confirms the diagnosis of perforation. Ref: Jones' Clinical Paediatric Surgery : Diagnosis and Management By Peter G. Jones, John M. Hutson, Alan A Woodward, Page 40; Walker's Pediatric Gastrointestinal Disease : Physiology, Diagnosis By W. Allan Walker, Volume 2, Page 1420