**Core Concept:** Meconium passage is a normal occurrence in newborns, typically within the first 24-48 hours postpartum. Vomiting and abdominal distension in this context may indicate intestinal obstruction, which can have various causes, including meconium ileus, Hirschsprung's disease, or intussusception.

**Why the Correct Answer is Right:** The correct answer, option D (barium enema), is appropriate for evaluating gastrointestinal (GI) obstruction in neonates. A barium enema involves the administration of an opaque contrast medium into the GI tract to visualize the anatomy and identify any obstructions or abnormalities. In this case, it helps to assess the extent and cause of the infant's GI obstruction, potentially leading to a definitive diagnosis and treatment plan.

**Why Each Wrong Option is Incorrect:**

A. X-ray (option A): While an X-ray can be helpful in some cases, barium enema offers a more detailed assessment of the GI tract and is more reliable for evaluating neonatal GI obstruction.

B. Ultrasound (option B): Ultrasound is a valuable diagnostic tool in neonates, but barium enema is more specific in evaluating GI obstruction due to its ability to visualize the entire GI tract and detect associated structural abnormalities.

C. Abdominal ultrasound (option C): Similar to option B, abdominal ultrasound is a crucial tool in neonatal diagnosis, but barium enema offers a more comprehensive evaluation of the GI tract and can detect additional structural abnormalities.

D. Clinical examination (option D): Clinical examination is essential in assessing the neonate's condition, but it is not sufficient for determining the cause and extent of GI obstruction. A diagnostic test is required, such as barium enema.

**Clinical Pearl:** Barium enema is a valuable diagnostic tool in evaluating neonatal GI obstruction, allowing for a more accurate diagnosis and treatment plan. Early intervention is crucial in these cases, as untreated neonatal GI obstruction can lead to serious complications, including malabsorption, electrolyte imbalances, and malnutrition.