## **Core Concept**
The appropriate size of an endotracheal tube (ETT) for a full-term infant is determined by the infant's age and size. The diameter and length of the ETT are crucial for ensuring proper ventilation and minimizing complications.

## **Why the Correct Answer is Right**
The correct formula for selecting the appropriate ETT size for a full-term infant involves using the following guidelines:
- The internal diameter (ID) of the ETT can be estimated using the formula: ( ID (mm) = (age : in : years + 4) / 4 ) for children, but for newborns, a more specific guideline is used.
- A commonly used estimate for full-term newborns is an ETT with an internal diameter of 3.0 mm and a length of 10-12 cm.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Incorrect because the dimensions provided do not match the standard guidelines for a full-term infant.
- **Option B:** Incorrect because, although closer, it still does not accurately represent the standard size for a full-term infant ETT.
- **Option D:** Incorrect because it suggests a smaller size that might be appropriate for preterm infants but not for full-term infants.

## **Clinical Pearl / High-Yield Fact**
A helpful mnemonic for estimating the ETT size in pediatric patients is that for a full-term newborn, an ETT with a 3.0 mm diameter is often used. Remembering that the length of the ETT for a full-term newborn is typically around 10-12 cm can help in clinical settings.

**Correct Answer: C. 3.0 mm 10 cm.**