**Core Concept**
Sleep apnea syndrome is a serious sleep disorder characterized by repeated episodes of partial or complete upper airway obstruction during sleep, leading to disrupted breathing patterns and reduced oxygen levels in the blood. This condition is often associated with obesity, smoking, and excessive alcohol consumption.

**Why the Correct Answer is Right**
The correct definition of sleep apnea syndrome involves the presence of repeated episodes of apnea or hypopnea, which are defined as complete or partial cessation of airflow for at least 10 seconds, or a 30% reduction in airflow for at least 10 seconds, respectively. These episodes can occur multiple times throughout the night, resulting in fragmented sleep and decreased oxygen saturation.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because sleep apnea syndrome does not exclusively involve episodes of complete upper airway obstruction. Hypopnea, a partial reduction in airflow, is also a critical component of the disorder.

**Option B:** Incorrect because sleep apnea syndrome can occur in individuals of normal body mass index (BMI). While obesity is a significant risk factor, it is not a prerequisite for the diagnosis.

**Option C:** Incorrect because sleep apnea syndrome is not solely defined by the presence of snoring. While snoring can be a symptom, it is not a diagnostic criterion.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that sleep apnea syndrome is a treatable condition, and diagnosis is often based on clinical history, physical examination, and overnight sleep studies (polysomnography). Treatment options include lifestyle modifications, continuous positive airway pressure (CPAP) therapy, and oral appliances.

**Correct Answer:** D.