Okay, let's tackle this question. The infant has a partial intestinal obstruction present from birth. The endoscopy didn't find an intraluminal lesion but showed narrowing in the mid duodenum. CT shows a mass surrounding that area. So, the key here is congenital and extrinsic causes of duodenal obstruction.

Mid duodenum narrowing with an extrinsic mass—that makes me think of annular pancreas. That's a congenital anomaly where pancreatic tissue surrounds the duodenum, causing obstruction. The endoscopy would see the narrowing, but the mass is outside the lumen, so no intraluminal lesion. Other possibilities could be malrotation with volvulus, but that usually presents with acute symptoms. Or maybe a duodenal atresia, but that's more proximal.

The CT showing a mass suggests something like pancreatic tissue. So annular pancreas would have pancreatic tissue in the lesion. Let me check the options. Wait, the options aren't listed here, but the correct answer is supposed to be pancreatic tissue. So the answer is pancreatic tissue. The other options might be things like lymph nodes, fibrous tissue, or other masses. But the classic presentation for annular pancreas is mid-duodenum narrowing with an extrinsic pancreatic mass. So the correct answer would be pancreatic tissue.

**Core Concept** The question assesses understanding of congenital causes of duodenal obstruction, focusing on annular pancreas—a condition where pancreatic tissue abnormally encircles the duodenum, causing extrinsic compression. This is a key differential for neonatal or infantile bowel obstruction.

**Why the Correct Answer is Right** Annular pancreas is a developmental anomaly where pancreatic tissue forms a ring-like structure surrounding the second portion of the duodenum. The extrinsic mass on CT and the absence of an intraluminal lesion on endoscopy align with this diagnosis. The narrowing at the mid-duodenum (second portion) is pathognomonic. Histologically, the mass comprises pancreatic acinar and ductal tissue, which can cause obstruction by mechanical compression.

**Why Each Wrong Option is Incorrect**
**Option A:** Lymph nodes—Lymphadenopathy is unlikely in a neonate with congenital obstruction and would not cause a localized annular mass.
**Option B:** Fibrous tissue—Strictures from fibrosis (e.g., post-inflammatory) are acquired and not congenital.
**Option C:** Adipose tissue—Lipomas or lipomatous tumors are rare in infants and do not cause annular compression.
**Option D:** Smooth muscle—Smooth muscle tumors (e.g., leiomyomas) are benign but not associated with annular pancreatic anomalies.

**Clinical Pearl** Annular pancreas is a classic cause of congenital duodenal obstruction, often diagnosed in neonates with bilious vomiting. Imaging (CT/MRI) reveals a "double bubble" sign on plain films and extrinsic pancreatic mass. Surgical management (duodenojejunostomy) is typically required.

**Correct Answer: D. Pancreatic tissue**

✓ Correct Answer: C. Pancreatic