WBR0558
| Author | PageAuthor::William J Gibson |
|---|---|
| Exam Type | ExamType::USMLE Step 1 |
| Main Category | MainCategory::Anatomy |
| Sub Category | SubCategory::Musculoskeletal/Rheumatology, SubCategory::General Principles |
| Prompt | [[Prompt::A 54-year-old man presents to an orthopedic surgeon with left shoulder weakness. The pain began suddenly when the patient attempted to lift a heavy box one month ago. While the pain has largely subsided, the patient is unable to initiate shoulder abduction. Physical exam reveals decreased muscle bulk on the superior aspect of the scapula on the left side compared to the right. Ultrasound reveals a torn tendon. Which of the following areas of the brachial plexus gives rise to the nerve that normally innervates the injured muscle in this patient?]] |
| Answer A | AnswerA::Axillary Nerve |
| Answer A Explanation | [[AnswerAExp::Incorrect: While the axillary nerve innervates the deltoid, an important shoulder abductor, the deltoid is not the most likely site of injury in this patient.
Musculocutaneous Nerve]] |
| Answer B | AnswerB::Musculocutaneous Nerve |
| Answer B Explanation | AnswerBExp::'''Incorrect:''' The musculocutaneous nerve is responsible for innervating the biceps. The injured muscle in this patient is the supraspinatus. |
| Answer C | AnswerC::Posterior Cord |
| Answer C Explanation | [[AnswerCExp::Incorrect: The posterior cord gives rise to the axillary and radial nerves. While the axillary nerve innervates the deltoid, an important shoulder abductor, the deltoid is not the most likely site of injury in this patient.]] |
| Answer D | AnswerD::Union of C5-C6 nerve roots |
| Answer D Explanation | AnswerDExp::'''Correct:''' The supraspinatus is responsible for initiating shoulder abduction and is innervated by the suprascapular nerve, which originates from the superior trunk of the brachial plexus (C5,C6). |
| Answer E | AnswerE::Union of C7-C8 nerve roots |
| Answer E Explanation | AnswerEExp::'''Incorrect:''' The union of the C7-C8 nerve roots represents the inferior trunk of the brachial plexus. |
| Right Answer | RightAnswer::D |
| Explanation | [[Explanation::The patient in this vignette has suffered a torn supraspinatus tendon. The supraspinatus is the most commonly injured muscle in rotator cuff injuries. The supraspinatus originates on the medial edge of the supraspinous fossa, where it proceeds to pass under the cover of the acromion to attach to the lateral aspect of the head of the humerus. The supraspinatus is responsible for initiating shoulder abduction and is innervated by the suprascapular nerve, which originates from the superior trunk of the brachial plexus (C5,C6). Contraction of the supraspinatus abducts the arm in the manner seen in the animation below.
Wiki-mnemonic: Rotator cuff muscles = SITS (Subscapularis, Infraspinatus, Teres minor, Suprascapularis). Educational Objective: The supraspinatus is responsible for initiating shoulder abduction and is innervated by the suprascapular nerve, which originates from the C5-C6 roots. References: First Aid 2012 page 405 |
| Approved | Approved::Yes |
| Keyword | WBRKeyword::Brachial, WBRKeyword::Brachial plexus, WBRKeyword::Shoulder, WBRKeyword::Arm, WBRKeyword::Upper limb, WBRKeyword::Rotator cuff, WBRKeyword::Tendon |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |