WBR0550

Author	[[PageAuthor::William J Gibson (Reviewed by Alison Leibowitz)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Anatomy
Sub Category	SubCategory::Musculoskeletal/Rheumatology
Prompt	[[Prompt::A 42-year-old male presents for a follow-up, after being treated for injuries from a motor vehicle collision 3 months earlier. The patient suffered a fracture of the surgical head of the left humerus, which was repaired surgically. One week prior, the arm was freed from immobilization, but the patient has not regained full motor function. Upon physical exam, the patient is unable to abduct the left arm more than 30 degrees from the body, but retains full function of the intrinsic muscles of the hand, and is able to move his wrist freely. Which of the following nerves of the brachial plexus is most likely injured in this patient? ]]
Answer A	AnswerA::A
Answer A Explanation	[[AnswerAExp::Point A indicates Erb’s point, the union of the C5 and C6 roots of the brachial plexus. Injury of the C5-C6 nerve roots is responsible for Erb’s palsy.]]
Answer B	AnswerB::B
Answer B Explanation	[[AnswerBExp::Point B represents the posterior cord of the brachial plexus, which gives rise to the axillary and radial nerves. While posterior cord injury would cause weakness of the deltoid muscle (through loss of axillary nerve fibers) the patient would most likely exhibit additional symptoms arising from loss of radial nerve function, such as wrist drop.]]
Answer C	AnswerC::C
Answer C Explanation	AnswerCExp::Point C represents the axillary nerve, which can be injured by fractures near the surgical head of the humerus. The axillary nerve innervates the deltoid muscle, which is responsible for abduction of the arm beyond 30 degrees from the body.
Answer D	AnswerD::D
Answer D Explanation	AnswerDExp::Point D represents the radial nerve. The radial nerve can be injured in conditions such as “Saturday night palsy”, in which the patient exhibit weakness of the wrist extensors.
Answer E	AnswerE::E
Answer E Explanation	AnswerEExp::Point E represents the union of the C8-T1 roots of the brachial plexus. This area can be injured in Krumpke’s palsy, causing a “total claw” deformity of the hand due to lumbrical dysfunction.
Right Answer	RightAnswer::C
Explanation	[[Explanation::The patient in this vignette has suffered a lesion of the axillary nerve, due to a fracture of the surgical head of the humerus. The axillary nerve derives from the C5-C6 roots of the brachial plexus and innervates the deltoid muscle. The deltoid is responsible for arm abduction more than 30 degrees from the body. Lesions of the axillary nerve cause a sensory deficit over the shoulder and eventually will cause a “flattening” of the shoulder due to atrophy of the deltoid. Educational Objective: Fractures of the surgical head of the humerus are a common cause of axillary nerve dysfunction. The axillary nerve is responsible for innervating the deltoid muscle. References: First Aid 2014 page 409.]]
Approved	Approved::Yes
Keyword	WBRKeyword::Brachial, WBRKeyword::Brachial plexus, WBRKeyword::Arm, WBRKeyword::Deltoid, WBRKeyword::nerve, WBRKeyword::innervation, WBRKeyword::muscular, WBRKeyword::nervous system
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::