This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Neuroanatomy, Subject name: Anatomy
Trismus is due to spasm of which muscle? 
Select and explain. 
A. Orbiculis, B. Lateral pterygoid, C. Mentalis, D. Medial pterygoid
D: Medial pterygoid
Answer: D: Medial pterygoid 
Explanation: 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.