WBR0426
| Author | [[PageAuthor::Rim Halaby, M.D. [1]]] |
|---|---|
| Exam Type | ExamType::USMLE Step 1 |
| Main Category | MainCategory::Pathology |
| Sub Category | SubCategory::Neurology |
| Prompt | [[Prompt::A 43 year old patient presents to his physician’s office for inability to differentiate between sweet and sour tastes at the tip of his tongue while eating his food. Upon further questioning, the patient denies inability to swallow. He 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 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 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 achieved by CN IX. In converse, salivation from the submandibular and sublingual glands is provided by CN VII. |
| Answer C | AnswerC::Inappropriate balance |
| Answer C Explanation | AnswerCExp::Balance is provided by 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 a reflex 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. On the other hand, sensation in the anterior 2/3 of the tongue is provided by the mandibular branch of the trigeminal nerve (CN V3). The taste and sensation of the posterior 2/3 of the tongue is provided by the glossopharyngeal nerve (CN IX). Motor innervation of the tongue is provided by the hypoglossal nerve (CN XII).
The patient in the vignette presents with symptoms of CN VII defect. CN VII innervates the stapedius muscle, the smallest skeletal muscle in the human body that is responsible of dampening vibrations of the stapes and prevents its excessive movement. Hyperacusis, the inability to tolerate normal sounds that are perceived as abnormally loud sounds, is caused by paralysis of the stapedius muscle due to CN VII injury that will cause very wide oscillation of the stapes and abnormally large reaction of the ossicles in response to normal sounds. Educational Objective:
CN VII, or facial nerve, innervates the anterior 2/3 of the tongue to give normal tongue sensation in that region. It also innervates the stapedius muscle in the ear that is responsible of dampening the vibration of the stapes in the ear. Injury of CNVII will cause inability to taste from the anterior 2/3 of the tongue and hyperacusis, the inability to tolerate normal sounds. |
| Approved | Approved::No |
| Keyword | WBRKeyword::Cranial, WBRKeyword::nerve, WBRKeyword::hyperacusis, WBRKeyword::tongue, WBRKeyword::taste |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |