A patient with trismus (limited mouth opening) most likely has which muscle involved?

Trismus (restricted mouth opening, typically less than 35-40 mm interincisal distance) most commonly results from spasm or involvement of the medial pterygoid muscle. The medial pterygoid runs from the medial surface of the lateral pterygoid plate to the medial surface of the mandibular ramus and is the primary jaw-closing muscle. It is particularly susceptible to spasm following inferior alveolar nerve blocks (due to needle trauma or hematoma formation), pericoronitis around lower wisdom teeth, or TMJ dysfunction. The masseter and temporalis also close the jaw and can contribute to trismus. The lateral pterygoid opens the jaw and would not cause trismus when affected.