WBR0426

Author	[[PageAuthor::Yazan Daaboul, M.D. (Reviewed by Yazan Daaboul, M.D. and Alison Leibowitz [1])]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Pathology
Sub Category	SubCategory::Neurology
Prompt	[[Prompt::A 43-year-old man presents to the physician’s office with complaints that he cannot differentiate between sweet and sour tastes at the tip of his tongue. Upon further questioning, the patient denies dysphagia, and reports he has a normal sensation of food with his tongue. The physician suspects the patient's symptoms are caused by an injury of a specific cranial nerve. Which of the following additional findings will further confirm the physician’s diagnosis?]]
Answer A	AnswerA::Inability to tolerate normal sounds
Answer A Explanation	[[AnswerAExp::Inability to tolerate normal sounds, or hyperacusis, may result from an injury to CN VII (facial nerve).]]
Answer B	AnswerB::Absence of salivation from the parotid gland
Answer B Explanation	AnswerBExp::Parotid gland is innervated by CN IX (glossopharyngeal nerve), while submandibular and sublingual glands are innervated by CN VII (facial nerve).
Answer C	AnswerC::Inability to maintain balance
Answer C Explanation	[[AnswerCExp::Inability to maintain balance may result from an injury to the vestibular component of CN VIII (vestibulocochlear nerve).]]
Answer D	AnswerD::Absence of pupillary constriction
Answer D Explanation	[[AnswerDExp::Pupillary constriction is provided by CN III.]]
Answer E	AnswerE::Absence of cough reflex
Answer E Explanation	[[AnswerEExp::Cough reflex is provided by CN X.]]
Right Answer	RightAnswer::A
Explanation	[[Explanation::The tongue is a complex structure that receives neural innervations from several cranial nerves (CN). The facial nerve (CN VII) is responsible for taste at the anterior 2/3 of the tongue, while sensation in the anterior 2/3 of the tongue is controlled by the mandibular branch of the trigeminal nerve (CN V3). The taste and sensation of the posterior 2/3 of the tongue is dependent on the glossopharyngeal nerve (CN IX). Motor innervation of the tongue is provided by the hypoglossal nerve (CN XII). The patient in this scenario presents with symptoms of an CN VII defect. CN VII innervates the stapedius muscle, the smallest skeletal muscle in the human body, which is responsible for dampening vibrations and preventing excessive movement of the stapes. Hyperacusis, the perception of normal sounds as abnormally loud, is caused by paralysis of the stapedius muscle due to an CN VII injury. This injury often causes a very wide oscillation of the stapes and an abnormally large reaction of the ossicles in response to normal sounds. Educational Objective: CN VII, the facial nerve, innervates the anterior 2/3 of the tongue, providing tongue sensation in that region. It also innervates the stapedius muscle in the ear, which is responsible for dampening the vibration of the stapes. Injury of CN VII frequently will cause an inability to taste from the anterior 2/3 of the tongue and hyperacusis. References: First Aid 2014 page 446]]
Approved	Approved::Yes
Keyword	WBRKeyword::Cranial, WBRKeyword::nerve, WBRKeyword::hyperacusis, WBRKeyword::tongue, WBRKeyword::taste, WBRKeyword::nerves, WBRKeyword::nervous system, WBRKeyword::muscles
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::