Step 1 topics → Musculoskeletal, Skin, and Connective Tissue → Brachial Plexus

Brachial Plexus

USMLE Step 1 trap: Reverses the root levels of Erb palsy (C5–C6) and Klumpke palsy (C8–T1). Erb palsy involves C5–C6 (upper trunk, 'waiter's tip'), while Klumpke palsy involves C8–T1 (lower trunk, intrinsic hand muscles).

The brachial plexus is one of the highest-yield anatomy topics on USMLE Step 1, and the Erb vs. Klumpke palsy distinction is the most reliably inverted: Erb palsy involves the upper trunk (C5–C6) from forced head-shoulder separation and produces the waiter's tip posture, while Klumpke palsy involves the lower trunk (C8–T1) from hyperabduction and destroys the intrinsic hand muscles. Flipping these costs you points on any identification question that gives you the mechanism. The plexus runs from C5–T1, reorganizes through roots → trunks → divisions → cords → branches, and each level maps to a distinct injury pattern. The exam tests pure recall, clinical vignette reverse-engineering, and passage-based reasoning on scapular winging, radial vs. axillary nerve distinctions, and thoracic outlet syndrome.

The trickiest part isn't memorizing the anatomy — it's keeping the clinical syndromes straight under pressure. Students consistently flip Erb and Klumpke palsy, misattribute scapular winging to the wrong nerve, and conflate radial with axillary nerve injuries just because both arise from the posterior cord. USMLE Step 1 exploits exactly these failure points, often embedding the correct nerve name but the wrong deficit, or describing a classic presentation without naming the nerve and asking you to identify it.

Thoracic outlet syndrome is another area where the exam catches students off guard. Most students fixate on the vascular presentation — swollen, discolored arm — but neurogenic TOS is far more common and presents subtly with intrinsic hand weakness and medial paresthesias. Knowing the difference, and knowing why (lower trunk / C8–T1 compression), separates students who truly own this topic from those who just memorized a table.

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Common misconceptions

Common mistake
Wrong: Erb palsy involves the lower trunk (C8–T1) and Klumpke involves the upper trunk (C5–C6).
Right: Erb palsy involves C5–C6 (upper trunk, 'waiter's tip'), while Klumpke palsy involves C8–T1 (lower trunk, intrinsic hand muscles).
Erb palsy results from forced separation of the head and shoulder — this stretches the upper trunk, which is C5–C6. The arm hangs in adduction and internal rotation with the forearm pronated: 'waiter's tip.' Klumpke palsy involves the lower trunk (C8–T1), typically from hyperabduction or a birth injury pulling the arm upward, and destroys the intrinsic hand muscles. A reliable anchor: Erb is the upper injury (baby's head pulled down at delivery), Klumpke is the lower injury (arm pulled up). Never flip these.
Common mistake
Wrong: Winging of the scapula is caused by axillary nerve injury.
Right: Scapular winging is caused by long thoracic nerve injury (C5–C7) leading to serratus anterior paralysis.
The long thoracic nerve (C5–C7) innervates the serratus anterior, which holds the scapula flat against the thorax. When it's injured — classically during axillary node dissection or a stab wound — the serratus fails and the medial border of the scapula protrudes posteriorly: that's winging. The axillary nerve has nothing to do with this; it innervates the deltoid and teres minor and produces lateral shoulder weakness and sensory loss, not scapular winging.
Common mistake
Wrong: Axillary nerve injury causes wrist drop because both the radial and axillary nerves arise from the posterior cord.
Right: Wrist drop is caused by radial nerve injury; axillary nerve injury causes deltoid weakness and loss of sensation over the lateral shoulder (regimental badge area).
Yes, both the radial and axillary nerves come from the posterior cord — but their injury patterns are completely different. Radial nerve injury (e.g., from a humeral shaft fracture) causes wrist drop because the radial nerve drives wrist and finger extensors. Axillary nerve injury (e.g., from a proximal humerus fracture or shoulder dislocation) causes deltoid weakness — the patient can't abduct the arm past 15 degrees — plus loss of sensation over the 'regimental badge' area of the lateral shoulder. Shared origin doesn't mean shared function.
Common mistake
Wrong: Thoracic outlet syndrome always presents with vascular symptoms (arm swelling, discoloration).
Right: Neurogenic thoracic outlet syndrome (compression of lower trunk/C8–T1) is far more common and presents with hand intrinsic muscle weakness and medial arm/forearm paresthesias.
Vascular TOS (subclavian vessel compression) is dramatic but rare. Neurogenic TOS is the common form and occurs when the lower trunk (C8–T1) is compressed, typically by a cervical rib or fibrous band. Because C8–T1 drives the intrinsic hand muscles (via the ulnar nerve) and provides sensation to the medial forearm and hand (via the medial cutaneous nerves), patients present with grip weakness, thenar or hypothenar wasting, and medial paresthesias — not a swollen arm. When USMLE Step 1 describes hand intrinsic wasting with medial paresthesias, think neurogenic TOS first.
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What the exam tests

  1. Know the five-level organization of the brachial plexus (roots, trunks, divisions, cords, branches) and which major nerves emerge from each cord — the exam will test which level a given nerve originates from.
  2. Identify the specific injury pattern for each major brachial plexus lesion: Erb palsy (C5–C6, waiter's tip), Klumpke palsy (C8–T1, intrinsic hand loss), long thoracic nerve injury (scapular winging), axillary nerve injury (deltoid weakness + lateral shoulder sensory loss), radial nerve injury (wrist drop), median nerve injury (ape hand / carpal tunnel), and ulnar nerve injury (claw hand / intrinsic weakness).
  3. Distinguish neurogenic from vascular thoracic outlet syndrome — recognize that neurogenic TOS (lower trunk / C8–T1 compression) is the common form and presents with hand intrinsic weakness and medial arm paresthesias, not primarily vascular signs.

Can you avoid these mistakes?

A 25-year-old motorcyclist falls and his head is forced laterally away from his right shoulder. He now holds his right arm adducted and internally rotated with the forearm pronated. Which roots are injured, and what is the eponym for this syndrome?
A surgeon performing a left axillary lymph node dissection inadvertently injures a nerve. Postoperatively, the patient's left scapula protrudes posteriorly when she pushes against a wall. Which nerve was injured, what muscle does it supply, and what is the root level?
A 35-year-old woman has progressive weakness of her intrinsic hand muscles and paresthesias along the medial forearm. Imaging reveals a cervical rib. What structure is being compressed, and why are the intrinsic hand muscles specifically affected?
A patient with a mid-shaft humerus fracture develops inability to extend his wrist. A classmate says this must be axillary nerve damage because both come from the posterior cord. Why is your classmate wrong, and what is the correct nerve and its expected sensory deficit?

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