**Core Concept**
The underlying principle being tested is the pathophysiology of paralytic ileus, a condition characterized by the temporary cessation of the normal contractions of the bowel muscles. This can occur post-infection, post-operatively, or due to various other causes. **Paralytic ileus** leads to bowel obstruction without any physical blockage.

**Why the Correct Answer is Right**
Given that the correct answer choices are not provided, a typical sign associated with paralytic ileus is abdominal distension due to the accumulation of gas and fluid within the intestines. The patient's history of recent pneumonia infection could be a precipitating factor, possibly leading to a systemic inflammatory response that affects bowel motility.

**Why Each Wrong Option is Incorrect**
**Option A:** Without the specific details of option A, it's challenging to provide a precise explanation for its incorrectness. However, if it were related to an active bowel sound, this would be incorrect because paralytic ileus is characterized by decreased or absent bowel sounds.
**Option B:** Similarly, without specifics, if option B suggested a high-pitched sound, this might be more indicative of a mechanical obstruction rather than paralytic ileus.
**Option C:** If option C mentioned a palpable mass, this could be incorrect as paralytic ileus does not typically present with a palpable abdominal mass.
**Option D:** If option D suggested hyperactive bowel sounds, this would be incorrect as paralytic ileus is associated with hypoactive or absent bowel sounds.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that paralytic ileus can be differentiated from mechanical obstruction by the presence of abdominal distension and the absence of high-pitched sounds or guarding. The condition often resolves with supportive care and bowel rest.

**Correct Answer:** D. Abdominal distension.