**Core Concept**
Bruxism refers to a parafunctional habit of teeth grinding or clenching, often occurring during sleep, which can lead to dental problems and facial pain. This condition is characterized by the abnormal and excessive movement of the mandible, resulting in the grinding or gnashing of teeth.

**Why the Correct Answer is Right**
The correct answer, grinding of teeth during sleep, accurately describes bruxism. During sleep, the brain's normal inhibitory mechanisms that regulate jaw movement are suppressed, leading to involuntary grinding or clenching of the teeth. This can be caused by various factors, including stress, anxiety, or abnormalities in the temporomandibular joint (TMJ). The grinding action can be so intense that it damages the teeth, leading to wear, chipping, or even fractures.

**Why Each Wrong Option is Incorrect**

**Option A:** Walking during sleep is known as somnambulism or sleepwalking, which is a distinct condition from bruxism. While both somnambulism and bruxism can occur during sleep, they involve different physiological mechanisms and are characterized by different symptoms.

**Option B:** Nocturnal enuresis, also known as bedwetting, is a condition where an individual urinates during sleep, often due to an immature bladder or an overactive bladder muscle. This condition is unrelated to bruxism and involves a different set of physiological mechanisms.

**Option D:** Sleep apnea is a condition where an individual experiences pauses in breathing during sleep, often due to obstruction of the airway or other factors. While sleep apnea can occur during sleep, it is not related to bruxism and is characterized by different symptoms, such as snoring, gasping for air, or excessive daytime sleepiness.

**Clinical Pearl / High-Yield Fact**
It's essential to note that bruxism can be a sign of an underlying psychological issue, such as anxiety or stress, and may also be linked to temporomandibular joint disorders (TMDs). As a clinician, being aware of the potential psychological and anatomical correlations of bruxism can help in providing effective treatment and management strategies.

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