If you’re preparing for the United States Medical Licensing Examination® (USMLE®) Step 1 exam, you might want to know which questions are most often missed by test-prep takers. Check out this example from Kaplan Medical, and read an expert explanation of the answer. Also check out all posts in this series.
This month’s stumper
A 31-year-old woman is brought to the emergency department after injuring her shoulder while playing soccer. Physical examination reveals an anterior dislocation of the glenohumeral joint.
The dislocation is reduced without complication. The patient returns because of significant, ongoing pain in the right arm and some weakness. She denies any sensory loss. Physical examination shows weakness of internal rotation of her right arm at the shoulder.
An injury in which of the following muscles is the most likely cause of this patient's symptoms?
A. Infraspinatus.
B. Pectoralis minor.
C. Subscapularis.
D. Supraspinatus.
E. Teres minor.
The correct answer is C.
Kaplan Medical explains why
The subscapularis muscle arises from the anterior surface of the scapula and inserts onto the lesser tubercle of the humerus. It is one of the rotator cuff muscles. Its tendon passes on the anterior side of the shoulder joint capsule, where it reinforces the capsule. Contraction of this muscle causes internal rotation of the arm at the shoulder. The subscapularis is innervated by the upper and lower subscapular nerves.
Why the other answers are wrong
Choice A: The infraspinatus muscle arises from the posterior surface of the scapula in the infraspinous fossa and inserts on the greater tubercle of the humerus. It is one of the rotator cuff muscles. Its tendon passes along the posterior surface of the shoulder joint capsule, where it reinforces the capsule. Contraction of the infraspinatus causes external rotation of the arm at the shoulder. The infraspinatus is innervated by the suprascapular nerve.
Choice B: The pectoralis minor muscle arises from the chest wall and inserts onto the coracoid process of the scapula. The pectoralis minor does not attach to the humerus and therefore does not cause movement of the humerus at the shoulder. It is not part of the rotator cuff. The pectoralis minor is innervated by the medial pectoral nerve.
Choice D: The supraspinatus muscle arises from the posterior surface of the scapula in the supraspinous fossa and inserts onto the greater tubercle. It is one of the rotator cuff muscles. Its tendon passes along the superior surface of the shoulder joint capsule, where it reinforces the capsule. Contraction of the supraspinatus causes abduction of the arm at the shoulder. It is most important in this function during the first 15-30 degrees of abduction. The supraspinatus is innervated by the suprascapular nerve.
Choice E: The teres minor muscle arises from the axillary border of the scapula and inserts onto the greater tubercle of the humerus. It is one of the rotator cuff muscles. Its tendon passes along the posterior surface of the shoulder joint capsule, where it reinforces the capsule. Contraction of the teres minor causes external rotation of the arm at the shoulder. The teres minor is innervated by the axillary nerve.
Tips to remember
- The four muscles of the rotator cuff include subscapularis, infraspinatus, supraspinatus, and teres minor.
- Acting together, they hold the head of the humerus in the glenoid fossa.
- Contraction of the subscapularis muscle causes internal rotation of the arm at the shoulder.
For more prep questions on USMLE Steps 1, 2 and 3, view other posts in this series.
The AMA and Kaplan have teamed up to support you in reaching your goal of passing the USMLE® or COMLEX-USA®. If you're looking for additional resources, Kaplan provides free access to tools for pre-clinical studies, including Kaplan’s Lecture Notes series, Integrated Vignettes, Shelf Prep and more.
