**Core Concept**
The presentation of bilious vomiting without bowel distension suggests an **obstructive** process, likely in the upper gastrointestinal tract. This is because bilious vomiting indicates that the obstruction is beyond the ampulla of Vater, where bile enters the intestine.

**Why the Correct Answer is Right**
Given the lack of bowel distension and absence of air-fluid levels on X-ray, the diagnosis is likely a **high intestinal obstruction**, such as pyloric stenosis or duodenal obstruction.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it would typically present with non-bilious vomiting.
**Option B:** Incorrect as it usually presents with abdominal pain and bloating.
**Option C:** Incorrect because it typically involves the lower gastrointestinal tract.

**Clinical Pearl / High-Yield Fact**
In cases of upper GI obstruction, the presence of bilious vomiting is a key differentiator from lower GI issues.

**Correct Answer:** D. Duodenal obstruction.