**Core Concept**
Intestinal obstruction occurs when there is a mechanical blockage in the intestines, preventing the normal flow of intestinal contents. This can lead to severe water loss due to the accumulation of fluid and electrolytes in the gut lumen.

**Why the Correct Answer is Right**
The correct answer is related to the concept of "obstruction site" and its impact on water loss. In the small intestine, there are specialized transport mechanisms that regulate water absorption. When an obstruction occurs in the small intestine, these mechanisms are disrupted, leading to a significant increase in water loss. This is because the small intestine is responsible for the majority of water absorption in the gastrointestinal tract. The site of obstruction is critical in determining the severity of water loss, with obstructions in the small intestine leading to more severe water loss compared to obstructions in the large intestine.

**Why Each Wrong Option is Incorrect**
**Option A:** Obstruction in the large intestine is less likely to result in severe water loss compared to obstruction in the small intestine, as the large intestine is primarily responsible for water absorption.
**Option B:** Obstruction at the pylorus (the region between the stomach and the small intestine) may lead to some water loss, but it is not as severe as obstruction in the small intestine.
**Option C:** Obstruction in the rectum is unlikely to result in significant water loss, as it is a terminal portion of the large intestine and water absorption is minimal in this region.

**Clinical Pearl / High-Yield Fact**
The severity of water loss in intestinal obstruction is directly related to the site of obstruction, with small intestine obstructions leading to more severe water loss due to impaired water absorption.

**Correct Answer:** C.