Trismus is due to spasm of which muscle?

Ans. D Medial pterygoidRef: Gray's Anatomy 41st ed. P 552* Trismus/Lock jaw is painful reflex muscle spasm. It occurs due to spasm of muscle of mastication and leads to inability to open the oral cavity* Trismus occurs due to involvement of masseter or medial pterygoid muscle or due to sub mucosal fibrosis.* Note: Infection of pterygomandibular region can cause trismus, which usually affects medial pterygoid muscle.