WBR0426
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| 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 complaining that he is unable to differentiate between sweet and sour tastes at the tip of his tongue. Upon further questioning, the patient denies dysphagia, and reports he has normal somatosensation of his tongue. The physician suspects the patient's symptoms are caused by an injury to 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 move the tongue |
| Answer C Explanation | AnswerCExp::Motor innervation to the tongue is provided by CN VII (hypoglossal nerve) |
| Answer D | AnswerD::Absence of pupillary constriction |
| Answer D Explanation | [[AnswerDExp::Pupillary constrictor muscles are innervated by the CN III (oculomotor nerve).]] |
| Answer E | AnswerE::Absence of cough reflex |
| Answer E Explanation | [[AnswerEExp::Absence of cough reflex may result from an injury to CN X (vagus nerve).]] |
| Right Answer | RightAnswer::A |
| Explanation | [[Explanation::The tongue is a complex structure that receives neural innervations from more than one cranial nerve (CN). The chorda tympani, which joins CN VII (facial nerve) as the latter exits the stylomastoid foramen, is responsible for the taste sensation in the anterior 2/3 of the tongue. On the other hand, somatosensation in the anterior 2/3 of the tongue is provided by the mandibular or 3rd branch of CN V (trigeminal nerve). The taste sensation and somatosensation of the posterior 2/3 of the tongue are both provided by CNIX (glossopharyngeal nerve). Motor innervation of the tongue is provided by CN XII (hypoglossal nerve). This patient presents with symptoms of a CN VII injury. CN VII innervates the stapedius muscle, which is responsible for dampening vibrations and preventing excessive oscillations of the stapes bone. Hyperacusis, the perception of normal sounds as abnormally loud, results from the paralysis of the stapedius muscle, which is often due to a CN VII injury. This injury 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 (facial nerve) innervates the anterior 2/3 of the tongue, which provides the taste sensation in that region. It also innervates the stapedius muscle in the ear, which is responsible for dampening the vibration of the stapes. CN VII injury should be suspected when patients present with complains of both inability to taste from the anterior 2/3 of the tongue and hyperacusis. |
| Approved | Approved::Yes |
| Keyword | WBRKeyword::Cranial nerve, WBRKeyword::Hyperacusis, WBRKeyword::Tongue, WBRKeyword::Taste, WBRKeyword::Nervous system, WBRKeyword::Facial nerve, WBRKeyword::CN VII, WBRKeyword::Somatosensation |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |