Okay, let's tackle this question. So, a 1-week-old infant with bilious vomiting. Bilious vomiting is a red flag in neonates, right? It usually means there's a bowel obstruction. The key here is the age and the symptom.

First, the core concept. In neonates, common causes of intestinal obstruction include things like intestinal atresia, malrotation with volvulus, or Hirschsprung's disease. But since the infant is only a week old, malrotation with volvulus is a big one. That's because the gut hasn't rotated properly during development, leading to a Ladd's bands causing obstruction, and the volvulus can cut off blood supply, leading to bilious vomiting.

Now, the correct answer. The question mentions an abdominal plain film and a barium enema. If the barium enema shows a "corkscrew" colon, that's classic for volvulus. Alternatively, if there's a "bubbly" appearance or a "string sign" on imaging, that might point to other issues. But in this case, the barium enema showing a corkscrew colon would support volvulus. So the most likely diagnosis here is malrotation with volvulus.

As for the other options, let's think. Intestinal atresia would present with bilious vomiting too, but it's more common in the first few days. The abdominal X-ray might show a "double bubble" sign if it's duodenal atresia. Hirschsprung's disease usually presents with chronic constipation, not acute vomiting. Intestinal obstruction from meconium plug might present with failure to pass meconium, but that's more in the first 24-48 hours.

The clinical pearl here is that bilious vomiting in a neonate is a surgical emergency. The key is to think of malrotation with volvulus and get an urgent imaging study. The barium enema can sometimes help in diagnosis and decompression before surgery. So the correct answer here is malrotation with volvulus, which would be option B, assuming the options are listed correctly.

**Core Concept**
Bilious vomiting in neonates <1 month is a surgical emergency, most commonly caused by **midgut volvulus** due to malrotation. This occurs when the duodenum and proximal jejunum twist around the superior mesenteric artery (SMA), leading to bowel obstruction and ischemia. The barium enema may show a "corkscrew" colon or "bubbly" appearance due to mesenteric vascular compression. **Why the Correct Answer is Right** Malrotation with volvulus is the leading cause of bilious vomiting in infants under 3 months. The twisting of the midgut around the SMA creates a **"whirlpool" sign** on imaging and a "string sign" on barium enema. The corkscrew colon appearance results from mesenteric vascular torsion. Immediate surgical intervention (Ladd’s procedure) is required to prevent bowel necrosis. **Why Each Wrong Option is Incorrect** **Option A:** *Duodenal atresia* presents with non-bilious vomiting and a "double bubble" sign on X-ray, not bilious vomiting. **Option C:** *Hirschsprung’s disease* causes

✓ Correct Answer: C. Malrotation with volvulus