## Core Concept
The passage of air through the gastrointestinal tract can be an indicator of intestinal patency and function. In infants, the time it takes for air to reach different parts of the gastrointestinal tract can provide valuable information about their gastrointestinal health.

## Why the Correct Answer is Right
The correct answer, , indicates that it takes approximately 30 minutes for air to reach the descending colon in a normal infant. This timeframe is consistent with the normal progression of gas through the gastrointestinal tract. After birth, gas typically enters the stomach within minutes, then progresses to the small intestine, and eventually reaches the large intestine.

## Why Each Wrong Option is Incorrect
* **Option A:** is too short for air to reach the descending colon, as it would not have enough time to pass through the stomach and small intestine.
* **Option B:** might seem plausible but is still too early for consistent arrival in the descending colon.
* **Option D:** is longer than necessary for air to reach the descending colon in a normal infant.

## Clinical Pearl / High-Yield Fact
A useful clinical correlation to remember is that in a normal infant, air should reach the stomach within 30 minutes to 1 hour after birth and the small intestine within 1-2 hours. This knowledge helps in assessing for intestinal obstruction or ileus.

**Correct Answer: C. 30 minutes.**