**Core Concept**
Bruxism refers to a sleep-related movement disorder characterized by the involuntary grinding, clenching, or gnashing of teeth during sleep. This condition can lead to various oral and dental complications, such as tooth wear, jaw pain, and temporomandibular joint (TMJ) disorders.

**Why the Correct Answer is Right**
The correct answer, grinding of teeth during sleep, accurately describes the primary symptom of bruxism. This grinding motion is often a result of excessive contraction of the masticatory muscles, including the masseter, temporalis, and medial pterygoid muscles. During sleep, these muscles can become overactive due to various factors, such as stress, anxiety, or sleep disorders, leading to the characteristic grinding or clenching of teeth.

**Why Each Wrong Option is Incorrect**
**Option A:** Walking during sleep is known as sleepwalking or somnambulism, which is a distinct sleep disorder characterized by the walking or performing complex behaviors while asleep. This is not related to bruxism.
**Option B:** Nocturnal enuresis refers to bedwetting, which is a common sleep disorder in children but is unrelated to bruxism. While both conditions occur during sleep, they involve different physiological mechanisms and symptoms.
**Option D:** Sleep apnea is a sleep disorder characterized by recurrent episodes of partial or complete upper airway obstruction during sleep, leading to pauses in breathing and potential cardiovascular complications. This condition is not directly related to bruxism, although both may be associated with sleep disorders.

**Clinical Pearl / High-Yield Fact**
Bruxism can be a symptom of underlying psychological or neurological conditions, such as anxiety, depression, or Parkinson's disease. Identifying and addressing these underlying factors is crucial for effective management and treatment of bruxism.

**✓ Correct Answer: C. Grinding of teeth during sleep**