A 43-year-old woman comes to the physician because of tingling and weakness in her left arm for the past 2 days. An image of the brachial plexus is shown. Nerve conduction study shows decreased transmission of electrical impulses in the labeled structure. Physical examination is most likely to show impairment of which of the following movements?

Extension of the wrist and fingers

Abduction of the shoulder above 100 degrees

Flexion of the metacarpophalangeal joints

A 61-year-old woman comes to the physician for a follow-up examination 1 week after undergoing right-sided radical mastectomy and axillary lymph node dissection for breast cancer. She says that she has been unable to comb her hair with her right hand since the surgery. Physical examination shows shoulder asymmetry. She is unable to abduct her right arm above 90 degrees. When she pushes against a wall, there is protrusion of the medial aspect of the right scapula. Injury to which of the following nerves is the most likely cause of this patient's condition?

Upper trunk of the brachial plexus

A 47-year-old woman presents to her primary care provider because of numbness and tingling on the palmar aspects of both hands. She denies any symptoms at the base of her thumbs. The symptoms are worse on the right (dominant hand) and are increased with activities such as driving or brushing her hair. She frequently wakes up with pain and has to shake her hand for pain relief. She has had rheumatoid arthritis for 9 years, for which she takes methotrexate. Her blood pressure is 124/76 mm Hg, the heart rate is 75/min, and the respiratory rate is 15/min. Lightly tapping over the middle of the anterior aspect of the right wrist leads to a tingling sensation in the palm. In this patient, electromyography (EMG) will most likely show which of the following results?

Focal slowing of conduction velocity in the median nerve in the carpal tunnel

Neuropathic changes in the palmar branch of the median nerve

Denervation in C7 innervated paraspinal, arms, and shoulder muscles

Widespread symmetrical neuropathic changes without focal abnormalities

Widespread denervation in proximal muscles with normal sensory nerves

A 35-year-old man is brought to the trauma bay by ambulance after sustaining a gunshot wound to the right arm. The patient is in excruciating pain and states that he can't move or feel his hand. The patient states that he has no other medical conditions. On exam, the patient's temperature is 98.4°F (36.9°C), blood pressure is 140/86 mmHg, pulse is 112/min, and respirations are 14/min. The patient is alert and his Glasgow coma scale is 15. On exam, he has a single wound on his right forearm without continued bleeding. The patient has preserved motor and sensation in his right elbow; however, he is unable to extend his wrist or extend his fingers further. He is able to clench his hand, but this is limited by pain. On sensory exam, the patient has no sensation to the first dorsal web space but has preserved sensation on most of the volar surface. Which of the following structures is most likely injured?

Recurrent motor branch of the median nerve

Upper limb cross-sections for USMLE Step 3: High-Yield Anatomy Lessons

Axilla Cross-Section - Armpit's Busy Junction\n\n\n\nKey structures organized around the axillary artery, all enclosed within the axillary sheath.\n\n* Neurovascular Bundle:\n - Axillary Artery: Central reference point.\n - Brachial Plexus Cords: Named by their position relative to the axillary artery (Lateral, Medial, Posterior).\n - Axillary Vein: Medial to the axillary artery.\n* Muscular Walls:\n - Anterior: Pectoralis major & minor.\n - Posterior: Subscapularis, Teres major, Latissimus dorsi.\n - Medial: Serratus anterior.\n - Lateral: Intertubercular sulcus of the humerus.\n\n> ⭐ The axillary sheath is a fascial sleeve enclosing the axillary artery, vein, and brachial plexus. Anesthetic injected here for an axillary block anesthetizes the terminal nerves (musculocutaneous, median, ulnar, radial) but often spares the axillary nerve.

Arm Cross-Section - Humeral Neighborhood Watch

Arm Cross-Section: Compartments & Neurovascular Bundles

Anterior (Flexor) Compartment: Bounded by the humerus and deep fascia.
- Muscles: Biceps brachii, Brachialis, Coracobrachialis.
- Neurovasculature: Musculocutaneous nerve (pierces coracobrachialis), Brachial artery, Median & Ulnar nerves.
Posterior (Extensor) Compartment:
- Muscle: Triceps brachii.
- Neurovasculature: Radial nerve & Profunda brachii artery (run together in the radial groove of the humerus).

⭐ High-Yield: The radial nerve is vulnerable to injury with mid-shaft humeral fractures, leading to "wrist drop" (inability to extend the wrist).

Forearm Cross-Section - Flex & Extend Central

Anterior Compartment (Flexor-Pronator):
- Action: Flexes wrist & digits; pronates forearm.
- Innervation: Median nerve (most) & Ulnar nerve (FCU, FDP medial half).
- Arterial Supply: Ulnar & Radial arteries.
Posterior Compartment (Extensor-Supinator):
- Action: Extends wrist & digits; supinates forearm.
- Innervation: Radial nerve (Posterior Interosseous Nerve).
- Arterial Supply: Posterior Interosseous artery.

⭐ The Posterior Interosseous Nerve (PIN) is vulnerable to entrapment within the supinator muscle (Arcade of Fröhse), leading to finger drop with preserved wrist extension (ECRL is spared).

Wrist Cross-Section - The Carpal Tunnel Squeeze

Wrist cross-section showing carpal tunnel contents

Boundaries of the Carpal Tunnel:
- Floor: Carpal bones (concave arch).
- Roof: Flexor retinaculum (transverse carpal ligament).
Contents (9 tendons, 1 nerve):
- Median nerve (most superficial, most easily compressed).
- Flexor pollicis longus tendon.
- 4 Flexor digitorum superficialis tendons.
- 4 Flexor digitorum profundus tendons.
📌 Mnemonic (Tendons): "She Likes To Play, Try To Catch Her" for carpal bones, but for tendons, remember 4+4+1 (FDS, FDP, FPL).

⭐ In Carpal Tunnel Syndrome, sensation over the thenar eminence is spared. The palmar cutaneous branch of the median nerve arises proximal to the wrist and travels superficial to the flexor retinaculum.

Brachial plexus cords are named based on their position relative to the axillary artery.

The anterior arm compartment contains the musculocutaneous nerve; the posterior compartment contains the radial nerve.

The radial nerve and profunda brachii artery travel together in the radial groove.

In the cubital fossa, the median nerve is medial to the brachial artery.

Median nerve supplies most anterior forearm flexors; radial nerve supplies all posterior extensors.

The ulnar nerve is vulnerable as it passes posterior to the medial epicondyle.

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