The most common cause of trismus in a dental patient is pericoronitis of the mandibular third molar. This condition involves inflammation of the soft tissues surrounding a partially erupted wisdom tooth, leading to pain, swelling, and spasm of the masticatory muscles. The infection and edema in the pericoronal flap restrict mandibular movement, resulting in trismus. Prompt management with local debridement, antibiotics, and possibly surgical extraction helps relieve symptoms and prevent progression. Other causes like TMJ dislocation, tetanus, and mumps are less common in the dental setting.

Reference: Oral and Maxillofacial Pathology, Neville, DD, 4th Edition.