## **Core Concept**
The question tests understanding of congenital causes of gastrointestinal obstruction, specifically those presenting in infancy with bilious vomiting and radiographic evidence of duodenal narrowing. The clinical presentation and imaging findings point towards a structural anomaly of the duodenum.

## **Why the Correct Answer is Right**
The correct answer, **Duodenal atresia or stenosis**, is a congenital condition characterized by complete or partial obstruction of the duodenum. This condition presents with bilious vomiting, failure to thrive, and an inability to feed adequately, consistent with the infant's symptoms. The upper GI series finding of marked narrowing of the midportion of the duodenum supports this diagnosis. Duodenal atresia or stenosis occurs due to a failure in the recanalization process of the duodenum during embryonic development.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is not provided, but typically, incorrect options might include conditions like pyloric stenosis, which presents with non-bilious vomiting and is more common in older infants.
- **Option B:** Similarly, another incorrect option could be gastroesophageal reflux disease (GERD), which causes non-bilious vomiting and does not typically present with duodenal narrowing on imaging.
- **Option C:** If an option like "jejunoileal atresia" were listed, it would be incorrect because, although it also causes intestinal obstruction, it typically presents with more distal bowel involvement and might not specifically cause narrowing of the midportion of the duodenum.
- **Option D:** If an option like "malrotation" were listed, it would be incorrect because, while it can cause duodenal obstruction, the imaging findings would more likely show a "corkscrew" appearance or an abnormal position of the duodenum rather than just narrowing.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is the "double bubble" sign on abdominal X-rays, which is classic for duodenal atresia or stenosis. This sign refers to the presence of two air-filled structures (the stomach and the duodenal bulb) with little to no air in the small intestine beyond the obstruction. This finding is highly suggestive of a duodenal obstruction and guides further diagnostic and therapeutic interventions.

## **Correct Answer:** . **Duodenal atresia or stenosis**