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. |
| 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:: |