WBR0551

Author	PageAuthor::William J Gibson
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Anatomy
Sub Category	SubCategory::Musculoskeletal/Rheumatology
Prompt	Prompt::An 18-year-old female softball player presents to her primary care physician with shoulder weakness. Which of the following muscles is responsible for the first 15 degrees of shoulder abduction?
Answer A	AnswerA::Deltoid
Answer A Explanation	AnswerAExp::While the deltoid is responsible for shoulder abduction, the deltoid is not responsible for initiating shoulder abduction. Instead, the deltoid primarily responsible for abduction greater than 30 degrees from the body.
Answer B	AnswerB::Infraspinatus
Answer B Explanation	AnswerBExp::The infraspinatus is a muscle of the rotator cuff that is responsible for lateral rotation of the arm. The infraspinatus and its tendon are commonly injured in baseball pitchers.
Answer C	AnswerC::Supraspinatus
Answer C Explanation	[[AnswerCExp::The supraspinatus is a muscle of the rotator cuff that is responsible for the first 15 degrees of shoulder abduction. The supraspinatus sits atop the scapula in the supraspinous fossa and its tendon proceeds through the under the acromion to attach to the lateral head of the humerus. Contraction of the supraspinatus abducts the arm in the manner seen in the animation below.]]
Answer D	AnswerD::Teres Major
Answer D Explanation	AnswerDExp::The teres major is large, flat muscle that medially rotates and adducts the humerus. It is innervated by the subscapular nerve (C5,C6).
Answer E	AnswerE::Teres Minor
Answer E Explanation	AnswerEExp::The teres minor is a rotator cuff muscle that is responsible for adduction and lateral rotation of the arm. It is innervated by the posterior branch of the axillary nerve (C5,C6).
Right Answer	RightAnswer::C
Explanation	[[Explanation::The patient in this vignette has injured her supraspinatus muscle, 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. Mnemonic: Rotator cuff muscles = SITS (Subscapularis, Infraspinatus, Teres minor, Suprascapularis). Educational Objective: The supraspinatus is responsible for initiating shoulder abduction. References: First Aid 2014 page 411]]
Approved	Approved::Yes
Keyword	WBRKeyword::Scapula, WBRKeyword::Brachial, WBRKeyword::Brachial plexus, WBRKeyword::Rotator cuff, WBRKeyword::Rotator, WBRKeyword::Cuff, WBRKeyword::Shoulder, WBRKeyword::Arm, WBRKeyword::Upper Limb
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::