**Question:** Trismus in parapharyngeal abscess is due to spasm of:

A. Masseter muscles
B. Temporalis muscles
C. Digastric muscles
D. Masticatory muscles

**Core Concept:**
Trismus is a clinical manifestation characterized by difficulty in opening the mouth due to muscle spasm. In the context of parapharyngeal abscess, trismus is caused by the inflammation and subsequent spasm of the masticatory muscles, which are responsible for jaw movement and mouth opening. The masticatory muscles are composed of four pairs: temporalis, masseter, digastric, and sternocleidomastoid.

**Why the Correct Answer is Right:**
In the setting of a parapharyngeal abscess, the infection can spread into the masticatory space, leading to inflammation and spasm of the masticatory muscles. Among the given options, only the masticatory muscles (option D) are involved in jaw movement and mouth opening, making them the correct answer.

**Why Each Wrong Option is Incorrect:**
Option A (Masseter muscles) are involved in jaw movement but not mouth opening. Option B (Temporalis muscles) are involved in jaw elevation but not mouth opening. Option C (Digastric muscles) are involved in the lateral movement of the mandible but not mouth opening. Option E (Sternocleidomastoid muscles) are involved in rotation and flexion of the head but not mouth opening.

**Clinical Pearl:**
Parapharyngeal abscesses should be suspected in patients with trismus as it can lead to serious complications if not promptly diagnosed and treated, such as sepsis, airway obstruction, and damage to adjacent structures. Early recognition and intervention are crucial to prevent these complications.

**Correct Answer:** D. Masticatory muscles