**Core Concept**
In acute intestinal obstruction, the initial clinical presentation is often a result of the obstruction's impact on bowel function and blood flow. The first sign to appear is typically related to the accumulation of fluid and gas in the intestine.

**Why the Correct Answer is Right**
The correct answer is the first symptom that appears due to the accumulation of fluid and gas in the intestine. This occurs because the obstruction prevents the normal flow of intestinal contents, leading to an increase in intraluminal pressure. As a result, fluid and gas accumulate in the intestine, causing the abdominal distension. The correct answer is related to the earliest clinical manifestation of this process.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because, although abdominal pain is a common symptom of acute intestinal obstruction, it is not the first to appear. Pain usually develops as the obstruction worsens and the bowel becomes more distended.
**Option B:** This option is incorrect because vomiting is a symptom of intestinal obstruction, but it typically appears later than the first symptom. Vomiting may occur as the obstruction worsens and the stomach becomes empty.
**Option C:** This option is incorrect because constipation is not typically the first symptom of acute intestinal obstruction. Constipation may occur if the obstruction is located in the lower part of the intestine, but it is not the earliest sign.

**Clinical Pearl / High-Yield Fact**
In acute intestinal obstruction, the initial clinical presentation often involves the accumulation of fluid and gas in the intestine, leading to abdominal distension. This is the earliest sign of the obstruction and should prompt further investigation.

**Correct Answer: A. Abdominal distension.**