**Core Concept**
Scalloping of the edge of the sigmoid colon on barium enema refers to the presence of rounded or scalloped projections on the margin of the colon, which can be indicative of a specific underlying pathology. This radiographic finding is often associated with changes in the intestinal wall or lumen.

**Why the Correct Answer is Right**
Pneumatosis intestinalis (PI) is a condition characterized by the presence of gas within the wall of the intestine, which can lead to the formation of rounded or scalloped projections on the barium enema. This occurs due to the accumulation of gas within the submucosal layer of the intestine, causing the mucosa to become irregular and scalloped. PI can be caused by various factors, including ischemia, infection, or inflammation.

**Why Each Wrong Option is Incorrect**
**Option A:** Diverticulosis is a condition characterized by the presence of diverticula, which are small, bulging pouches in the wall of the colon. While diverticulosis can cause changes in the colon's appearance, it typically results in the formation of multiple small diverticula rather than scalloped projections.

**Option B:** Ulcerative colitis is a type of inflammatory bowel disease that can cause changes in the colon's appearance, including loss of haustra, shortening, and narrowing. While ulcerative colitis can lead to irregularities in the colon's surface, it is not typically associated with the scalloping seen on barium enema in pneumatosis intestinalis.

**Option C:** Carcinoma colon refers to cancer of the colon, which can cause a range of changes in the colon's appearance, including narrowing, obstruction, and irregularities in the colon's surface. However, carcinoma colon is not typically associated with the characteristic scalloping seen on barium enema in pneumatosis intestinalis.

**Clinical Pearl / High-Yield Fact**
When interpreting barium enema studies, it's essential to recognize that pneumatosis intestinalis can be a sign of underlying ischemia or infection. This condition requires prompt medical attention, and prompt recognition of the characteristic scalloping on barium enema can help guide further management.

**✓ Correct Answer: D. Pneumatosis intestinalis**