**Core Concept**
Neonatal necrotizing enterocolitis (NEC) is a condition characterized by inflammation and necrosis of the intestinal tissue, primarily affecting premature infants. The radiological findings in NEC are critical for its diagnosis and monitoring. In this context, the presence of gas within the intestinal wall is a hallmark feature.

**Why the Correct Answer is Right**
The correct answer, **Gas in the intestinal wall**, is indicative of pneumatosis intestinalis, a condition where gas accumulates within the intestinal wall due to bacterial fermentation of intestinal contents. This is a result of the breakdown of the intestinal mucosa, allowing bacteria to penetrate the wall and produce gas. Pneumatosis intestinalis is a specific radiological feature of NEC and is often seen on abdominal radiographs. The presence of gas in the intestinal wall is a result of the intestinal mucosa's loss of integrity, allowing bacteria to penetrate the wall and produce gas.

**Why Each Wrong Option is Incorrect**
**Option A:** Gas in the portal system is not a characteristic finding in NEC. While portal venous gas can be seen in conditions such as intestinal ischemia, it is not a specific feature of NEC.

**Option C:** Pneumoperitonium, or free air in the peritoneal cavity, is not a specific finding in NEC. While it can occur in NEC due to perforation of the intestine, it is not a characteristic feature of the condition.

**Option D:** Air-fluid levels are a common finding in conditions such as intestinal obstruction or ileus, but are not specific to NEC.

**Clinical Pearl / High-Yield Fact**
In diagnosing NEC, it's essential to remember that pneumatosis intestinalis is a specific radiological feature that, in the context of clinical presentation, can help differentiate NEC from other conditions. Additionally, the presence of pneumatosis intestinalis on abdominal radiographs should prompt a thorough evaluation for NEC, particularly in premature infants.

**✓ Correct Answer: B. Gas in the intestinal wall**