**Core Concept**
Masticator space infection is a type of odontogenic infection that occurs in the masticator space, which is a fascial compartment containing the muscles of mastication (chewing). This infection often originates from dental sources and can spread to the surrounding tissues.

**Why the Correct Answer is Right**
The correct answer is usually an infection of the tooth or dental procedure. The masticator space is a potential space that contains the masseter muscle, medial pterygoid muscle, lateral pterygoid muscle, and the buccinator muscle. Infections in this space often arise from the teeth, particularly the lower molars and premolars, which are located near the masticator space. The infection can spread to the masticator space through the alveolar bone, resulting in swelling, pain, and trismus (difficulty opening the mouth).

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because while the parotid gland is a salivary gland located near the masticator space, infections of the parotid gland typically result from sialadenitis (inflammation of the salivary gland) rather than odontogenic sources.
* **Option B:** This option is incorrect because while the submandibular space is a potential space that contains lymph nodes and salivary glands, infections in this space are more commonly related to Ludwig's angina (bilateral submandibular space infections) rather than masticator space infections.
* **Option C:** This option is incorrect because while the pterygoid venous plexus is a venous structure that drains blood from the masticator space, it is not a common source of infection for this space.

**Clinical Pearl / High-Yield Fact**
Masticator space infections can be life-threatening if left untreated, as they can lead to complications such as airway obstruction, mediastinitis, and sepsis. Prompt diagnosis and treatment, often involving antibiotics and surgical drainage, are essential to prevent these complications.

**Correct Answer:** D.