**Core Concept**
The size of an endotracheal tube (ETT) in a full-term infant is determined by the internal diameter of the tube, which is typically measured in millimeters (mm). The correct size of the ETT is crucial for ensuring adequate ventilation and minimizing complications such as tracheal damage or obstruction.

**Why the Correct Answer is Right**
In full-term infants, the internal diameter of the endotracheal tube should be approximately 3.5 to 4.0 mm. This size is based on the average size of the infant's trachea, which is about 4.5 to 5.5 mm in diameter. Using a tube that is too small can lead to inadequate ventilation, while a tube that is too large can cause tracheal damage or obstruction. The correct size of the tube is crucial for ensuring adequate ventilation and minimizing complications.

**Why Each Wrong Option is Incorrect**
**Option A:** 2.5 mm - This size is too small for a full-term infant, and would likely lead to inadequate ventilation.
**Option B:** 5.5 mm - This size is too large for a full-term infant, and would increase the risk of tracheal damage or obstruction.
**Option C:** 6.0 mm - This size is also too large for a full-term infant, and would increase the risk of tracheal damage or obstruction.

**Clinical Pearl / High-Yield Fact**
When selecting an endotracheal tube size for a full-term infant, it is essential to consider the patient's weight and gestational age, as well as the size of the trachea. A general rule of thumb is to use a tube that is 3.5 to 4.0 mm in internal diameter for full-term infants.

**Correct Answer:** C. 4.0 mm