**Core Concept**
Trismus, also known as lockjaw, is a condition characterized by a reduced opening of the mouth due to spasms of the muscles of mastication. This condition can be caused by various factors, including infections, inflammation, and trauma. The primary muscle responsible for this condition is the medial pterygoid muscle, which plays a crucial role in the movement of the mandible.

**Why the Correct Answer is Right**
The medial pterygoid muscle is one of the muscles of mastication that helps in the opening and closing of the jaw. When this muscle goes into spasm, it can lead to trismus, making it difficult to open the mouth. The medial pterygoid muscle is innervated by the mandibular division of the trigeminal nerve (V3), which is responsible for the motor function of the muscles of mastication. The spasm of the medial pterygoid muscle can be caused by various factors, including infections such as otitis media or abscesses in the region, which can irritate the nerve and lead to muscle spasm.

**Why Each Wrong Option is Incorrect**
**Option A:** The temporalis muscle is another muscle of mastication that helps in the elevation of the mandible, but it is not primarily responsible for trismus. While it can contribute to the reduction of the jaw opening, it is not the primary muscle involved.
**Option B:** The masseter muscle is another muscle of mastication that helps in the elevation of the mandible, but it is not primarily responsible for trismus. It can contribute to the reduction of the jaw opening, but it is not the primary muscle involved.
**Option C:** The digastric muscle is involved in the movement of the hyoid bone and the mandible, but it is not primarily responsible for trismus. It can contribute to the reduction of the jaw opening, but it is not the primary muscle involved.

**Clinical Pearl / High-Yield Fact**
It's essential to note that trismus can be a sign of a more serious underlying condition, such as a dental infection or an abscess in the region. Therefore, if a patient presents with trismus, it's crucial to investigate the underlying cause and provide appropriate treatment to prevent further complications.

**Correct Answer: C. Masseter muscle.**