<prompt> Trismus is due to spasm of which muscle?OrbiculisLateral pterygoidMentalisMedial pterygoid  <response>: Ans. D Medial pterygoidRef: Grays Anatomy 41st ed. P 552 TrismusLock 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. 