A patient cannot control saliva and drools from one corner of the mouth. Loss of function of which muscle is most directly responsible?

Correct answer: B. Orbicularis oris

• A. The masseter elevates the mandible (CN V3); its loss impairs chewing power, not lip seal.
• B. CORRECT. ORBICULARIS ORIS is the sphincter muscle encircling the mouth and is responsible for competent LIP SEAL. It is a muscle of facial expression innervated by the FACIAL NERVE (CN VII). When CN VII function is lost — Bell palsy, stroke, or surgical injury — the lip seal fails on the affected side and saliva escapes. This is why drooling and inability to purse the lips or whistle are classic signs of facial nerve palsy.
• C. The temporalis elevates and retracts the mandible (CN V3).
• D. The medial pterygoid elevates and protrudes the mandible (CN V3).

Note: Muscles of MASTICATION = CN V3, they move the jaw. Muscles of EXPRESSION = CN VII, they move the face and seal the lips.