WBR0426
| Author | [[PageAuthor::Rim Halaby, M.D. [1] (Reviewed by Alison Leibowitz)]] |
|---|---|
| Exam Type | ExamType::USMLE Step 1 |
| Main Category | MainCategory::Pathology |
| Sub Category | SubCategory::Neurology |
| Prompt | [[Prompt::A 43-year-old male presents to the physician’s office with complaints of an inability to differentiate between sweet and sour tastes at the tip of his tongue. Upon further questioning, the patient denies inability to swallow and reports normal sensation of food in his tongue. The physician suspects an injury of a specific cranial nerve (CN) might be causing the patient’s symptoms. 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::the inability to tolerate normal sounds, or hyperacusis, may be caused by an injury to CN VII.]] |
| Answer B | AnswerB::Absence of salivation from the parotid gland |
| Answer B Explanation | AnswerBExp::Salivation from the parotid gland is dependent on CN IX, while salivation from the submandibular and sublingual glands is provided by CN VII. |
| Answer C | AnswerC::Inappropriate balance |
| Answer C Explanation | [[AnswerCExp::Balance is dependent on CN VIII.]] |
| 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:: |