**Core Concept**
Monitoring apnea in preterm infants is crucial for early detection and intervention. Apnea of prematurity is a common condition characterized by episodes of breathing pause lasting ≥ 20 seconds or ≤ 8-10 breaths per minute in preterm infants.

**Why the Correct Answer is Right**
The best method to monitor apnea in preterm infants is by using **transcutaneous oxygen saturation monitoring** (option C). This non-invasive method uses a sensor placed on the skin to measure oxygen saturation levels, which can help detect episodes of apnea and bradycardia. The sensor provides real-time data, allowing for prompt intervention.

**Why Each Wrong Option is Incorrect**
* **Option A:** Infrared pulse oximetry is used to measure oxygen saturation, but it's not the best method for detecting apnea.
* **Option B:** ECG monitoring is used to detect cardiac arrhythmias but may not accurately detect apnea episodes.

**Clinical Pearl / High-Yield Fact**
Preterm infants with apnea of prematurity are at risk for sudden infant death syndrome (SIDS). Monitoring and managing apnea can significantly reduce this risk.

**Correct Answer: C. Transcutaneous oxygen saturation monitoring.**