**Core Concept**
Bruxism refers to a sleep-related movement disorder characterized by grinding or clenching of the teeth, typically during sleep or in response to stress and anxiety. It involves the masticatory muscles, including the masseter, temporalis, and medial pterygoid muscles.

**Why the Correct Answer is Right**
The correct answer involves the pathophysiology of bruxism, which is often linked to stress, anxiety, and other psychological factors. The masticatory muscles, particularly the masseter, are involved in the grinding and clenching movements. In response to stress or anxiety, the brain may trigger the masticatory muscles to contract, leading to bruxism. This is often seen in individuals with a history of anxiety disorders, depression, or other mental health conditions.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as bruxism is not typically associated with a specific age group, although it may be more common in certain demographics.
* **Option B:** This option is incorrect as bruxism is not directly related to a specific type of medication, although certain medications may exacerbate the condition.
* **Option C:** This option is incorrect as bruxism is not primarily a sleep disorder, although it may occur during sleep; it is more accurately described as a movement disorder.

**Clinical Pearl / High-Yield Fact**
A common clinical pearl is that bruxism is often associated with temporomandibular joint (TMJ) disorders, such as TMJ pain and clicking. This is because the masticatory muscles are involved in both bruxism and TMJ disorders.

**Correct Answer: D. Anxiety disorders**