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 56-year-old man presents to the emergency room after being in a motor vehicle accident. He was driving on an icy road when his car swerved off the road and ran head on into a tree. He complains of severe pain in his right lower extremity. He denies loss of consciousness during the accident. His past medical history is notable for poorly controlled hypertension, hyperlipidemia, and major depressive disorder. He takes enalapril, atorvastatin, and sertraline. His temperature is 99.1°F (37.3°C), blood pressure is 155/85 mmHg, pulse is 110/min, and respirations are 20/min. On exam, he is alert and fully oriented. He is unable to move his right leg due to pain. Sensation is intact to light touch in the sural, saphenous, tibial, deep peroneal, and superficial peroneal distributions. His leg appears adducted, flexed, and internally rotated. An anteroposterior radiograph of his pelvis would most likely demonstrate which of the following findings?

Femoral head smaller than contralateral side and posterior to acetabulum

Fracture line extending between the greater and lesser trochanters

Femoral head larger than contralateral side and inferior to acetabulum

Fracture line extending through the femoral neck

Fracture line extending through the subtrochanteric region of the femur

A 30-year-old male presents with pain and limited movement in his shoulder following a fall. X-ray reveals an anterior dislocation of the glenohumeral joint. Which of the following structures is most likely to be damaged in this injury?

Anterior inferior glenohumeral ligament

Comparison of upper and lower limb structures for USMLE Step 1: High-Yield Anatomy Lessons

Homology & Development - Blueprint Buddies

Shared Origin: Both limbs arise from lateral plate mesoderm as buds around week 4, directed by Hox genes.
Opposing Rotation: Limbs rotate 90° in opposite directions.
- Upper Limb: Rotates laterally (externally).
- Lower Limb: Rotates medially (internally).
Result: This establishes the final orientation of flexor/extensor compartments. The elbow points posteriorly, while the knee points anteriorly.

⭐ The opposite 90° rotation of the limb buds is a key developmental event that explains the final anatomical arrangement of muscles and dermatomes.

Embryonic limb development and rotation

Bones & Joints - Mobility vs. Stability

The upper limb is optimized for mobility, while the lower limb is built for stability and weight-bearing. This fundamental difference is reflected in their bony and articular architecture.

Feature	Upper Limb (Mobility)	Lower Limb (Stability)
Primary Joint	Glenohumeral (Shoulder)	Acetabulofemoral (Hip)
Socket	Shallow glenoid fossa	Deep acetabulum
Range of Motion	Wide, multi-axial	More restricted
Ligaments	Relatively lax	Strong, reinforcing
Primary Role	Manipulation, reach	Weight-bearing, locomotion

⭐ The glenohumeral joint's extreme mobility comes at a cost: it is the most commonly dislocated large joint in the body.

Neurovascular Supply - Life Support Lines

Arterial Axis: A single major artery supplies each limb, branching distally.
Nerve Plexus: A network of nerves from spinal roots innervates each limb.

Arterial supply of upper limbs from aortic arch

Feature	Upper Limb	Lower Limb
Artery	Subclavian → Axillary → Brachial → Radial/Ulnar → Palmar Arches	External Iliac → Femoral → Popliteal → Ant/Post Tibial → Plantar Arch
Plexus	Brachial Plexus (C5-T1)	Lumbosacral Plexus (L1-S4)
Key Nerves	Musculocutaneous, Axillary, Radial, Median, Ulnar	Femoral, Obturator, Sciatic (Tibial & Common Peroneal)

⭐ Exam Favorite: The common peroneal (fibular) nerve is the most frequently injured nerve in the lower leg, especially vulnerable to fracture of the fibular neck. This can lead to foot drop.

Clinical Correlates - When Things Go Wrong

Joint Instability & Injury:
- Upper Limb: Shoulder dislocation (most commonly anterior) due to extreme mobility. Rotator cuff tears are frequent.
- Lower Limb: Hip dislocation (most commonly posterior) from major trauma. Anterior Cruciate Ligament (ACL) tears are common knee injuries.
Common Fractures & Avascular Necrosis (AVN) Risk:
- Upper Limb: Scaphoid fracture at the wrist carries a high risk of AVN.
- Lower Limb: Femoral neck fracture at the hip endangers the femoral head with AVN.

⭐ The shoulder's mobility makes it prone to anterior dislocation, whereas the hip's stability means dislocation is typically posterior and requires significant force.

Upper limb is for mobility and manipulation; lower limb is for stability and locomotion.

The shoulder joint has a wide range of motion but is unstable; the hip joint is stable with less mobility.

During development, the upper limb rotates laterally while the lower limb rotates medially.

Homologous bones include the humerus/femur and carpals/tarsals.

Innervation is from the brachial plexus (C5-T1) versus the lumbosacral plexus (L1-S4).

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