A 25-year-old man comes to the physician because of a 2-week history of numbness in his left lower extremity. One month ago, he sustained a fracture of the neck of the left fibula during soccer practice that was treated with immobilization in a plaster cast. Physical examination of the left lower extremity is most likely to show which of the following findings?

Impaired dorsiflexion of the foot

Loss of sensation over the medial calf

Loss of sensation on the sole of the foot

A 51-year-old woman comes to the physician because of progressively worsening lower back pain. The pain radiates down the right leg to the lateral side of the foot. She has had no trauma, urinary incontinence, or fever. An MRI of the lumbar spine shows disc degeneration and herniation at the level of L5–S1. Which of the following is the most likely finding on physical examination?

Exaggerated patellar tendon reflex

Diminished sensation of the anus and genitalia

Diminished sensation of the anterior lateral thigh

A 57-year-old man presents to the ED complaining of back and left leg pain. He was lifting heavy furniture while helping his daughter move into college when all of sudden he felt a sharp pain at his back. The pain is described as severe, worse with movement, and shoots down his lateral thigh. The patient denies any bowel/urinary incontinence, saddle anesthesia, weight loss, or weakness. He denies any past medical history but endorses a family history of osteoporosis. He has been smoking 1 pack per day for the past 20 years. Physical examination demonstrated decreased sensation at the left knee, decreased patellar reflex, and a positive straight leg test. There is diffuse tenderness to palpation at the lower back but no vertebral step-offs were detected. What is the most likely etiology for this patient’s pain?

Disc herniation at the L3/L4 vertebra

Disc herniation at the L4/L5 vertebra

Spinal metastasis from lung cancer

Segmental innervation patterns for USMLE Step 1: High-Yield Anatomy Lessons

Dermatomes & Myotomes - The Body's Wiring

Dermatome: Area of skin supplied by a single spinal nerve root.
Myotome: Group of muscles supplied by a single spinal nerve root.

Key Landmarks:

C4: Shoulder cape
T4: Nipple line (📌 T-4 = Teat-four)
T10: Umbilicus (📌 T-10 = Bellybu-ten)
L4: Knee cap
S2-S4: Perineum

Key Myotome Actions:

C5: Shoulder abduction
C7: Elbow extension
L5: Big toe extension
S1: Plantarflexion

Dermatome map with spinal nerve roots

⭐ Herpes zoster (shingles) reactivates in a dorsal root ganglion, causing a painful, unilateral vesicular rash restricted to a single dermatome.

Upper Limb Innervation - Arming the Nerves

Upper Limb Dermatomes, Myotomes, and Nerve Innervation

Key Myotomes (Motor) & Reflexes:
- C5: Deltoid (abduction), Biceps (flexion) → Biceps reflex
- C6: Wrist extension → Brachioradialis reflex
- C7: Triceps (extension), Wrist flexion → Triceps reflex
- C8: Finger flexion (grip)
- T1: Finger abduction (interossei)
Key Dermatomes (Sensory):
- C5: Lateral shoulder (deltoid badge area)
- C6: Lateral forearm, thumb, index finger
- C7: Middle finger
- C8: Medial forearm, ring, little finger
- T1: Medial arm/axilla

⭐ Erb-Duchenne Palsy: Injury to C5-C6 roots (upper trunk) results in a "waiter's tip" hand: adducted, internally rotated arm with an extended elbow.

Lower Limb Innervation - Leg Day Nerves

Femoral Nerve (L2-L4):
- Motor: Hip flexion (iliopsoas), knee extension (quadriceps).
- Sensory: Anterior thigh, medial leg.
- Reflex: Patellar (L4).
Obturator Nerve (L2-L4):
- Motor: Thigh adduction.
- Sensory: Medial thigh.
Sciatic Nerve (L4-S3): Splits into tibial and common peroneal.
- Tibial (L4-S3): Plantarflexion, inversion. Sensation to sole.
- Common Peroneal (L4-S2): Dorsiflexion, eversion. Sensation to anterolateral leg & dorsum of foot.

📌 Mnemonic: "L4, kick the door" (knee extension); "S1, point to the sun" (plantarflexion).

Sensory Innervation of the Sciatic Nerve

⭐ High-Yield: Injury to the common peroneal nerve (e.g., fibular neck fracture) causes foot drop: inability to dorsiflex (L5) and evert the foot, leading to a high-stepping gait.

Clinical Correlations - Pinched & Pointless

Cervical & Lumbar Disc Herniation & Nerve Root Compression

Radiculopathy ("Pinched Nerve"): Nerve root compression, typically from a herniated disc or spondylosis, causing segmental deficits.
- Dermatomal Signs: Radiating pain (sciatica), paresthesia ("pins & needles"), or sensory loss in a specific skin distribution.
- Myotomal Signs: Weakness in specific muscle groups (e.g., foot drop with L5).
- Reflexes: Diminished or absent deep tendon reflexes (DTRs) corresponding to the affected nerve root (e.g., ↓ Achilles reflex with S1 compression).

⭐ A positive Straight Leg Raise (Lasègue's sign) is highly suggestive of lumbar radiculopathy, typically from L4-S1 disc herniation.

High‑Yield Points - ⚡ Biggest Takeaways

C3-C5 innervates the diaphragm (phrenic nerve), essential for breathing.

C5 controls shoulder abduction (deltoid); C6 manages wrist extension and the biceps reflex.

C7 handles wrist flexion and finger extension, tested by the triceps reflex.

L4 is key for knee extension via the patellar reflex and sensation over the medial malleolus.

S1 mediates plantarflexion (Achilles reflex) and sensation on the lateral foot.

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