**Core Concept:**
Hypopnea is a clinical term used to describe a partial reduction in the amount of air entering the lungs during sleep, leading to a decrease in oxygen saturation and increase in carbon dioxide levels. It is a common respiratory disorder seen in sleep apnea patients.

**Why the Correct Answer is Right:**
Hypopnea is defined as a reduction in airflow by at least 30% and by more than 50% of the individual's baseline airflow for at least 10 seconds during sleep, accompanied by a decrease in oxygen saturation of at least 3%. This definition ensures that the event is significant enough to cause an impact on oxygen saturation but not severe enough to qualify as a central or obstructive apnea.

**Why Each Wrong Option is Incorrect:**
A. This option is incorrect because a reduction in airflow by less than 30% does not meet the criteria for hypopnea.
B. This option is incorrect because a reduction in airflow by more than 50% would qualify as an obstructive apnea, not hypopnea.
C. This option is incorrect as it only requires a reduction in airflow without considering the accompanying oxygen saturation decrease.
D. This option is incorrect as it does not specify the percentage reduction in airflow and oxygen saturation, making the definition incomplete.

**Clinical Pearl / High-Yield Fact:**
Hypopnea and apnea are related terms, but they differ in severity. Hypopnea involves a partial reduction in airflow, while apnea refers to a complete absence of airflow. Understanding the distinction between these terms is crucial for diagnosing and managing sleep disorders effectively.

**Correct Answer:** C. Reduction in airflow by more than 50% of the individual's baseline airflow for at least 10 seconds during sleep, accompanied by a decrease in oxygen saturation of at least 3%