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 23-year-old woman presents to the emergency department with an acute exacerbation of her 3-month history of low back and right leg pain. She says she has had similar symptoms in the past, but this time the pain was so excruciating, it took her breath away. She describes the pain as severe, shock-like, and localized to her lower back and radiating straight down the back of her right thigh and to her calf, stopping at the ankle. Her pain is worse in the morning, and, sometimes, the pain wakes her up at night with severe buttock and posterior thigh pain but walking actually makes the pain subside somewhat. The patient reports no smoking history or alcohol or drug use. She has been working casually as a waitress and does find bending over tables a strain. She is afebrile, and her vital signs are within normal limits. On physical examination, her left straight leg raise test is severely limited and reproduces her buttock pain at 20° of hip flexion. Pain is worsened by the addition of ankle dorsiflexion. The sensation is intact. Her L4 and L5 reflexes are normal, but her S1 reflex is absent on the right side. A CT of the lumbar spine shows an L5–S1 disc protrusion with right S1 nerve root compression. Which of the following muscle-nerve complexes is involved in producing an S1 reflex?

Gastrocnemius/soleus-tibial nerve

Tibialis posterior-tibial nerve

Quadriceps femoris-femoral nerve

A 28-year-old male presents to his primary care physician with complaints of intermittent abdominal pain and alternating bouts of constipation and diarrhea. His medical chart is not significant for any past medical problems or prior surgeries. He is not prescribed any current medications. Which of the following questions would be the most useful next question in eliciting further history from this patient?

"Can you tell me more about the symptoms you have been experiencing?"

"Does the diarrhea typically precede the constipation, or vice-versa?"

"Is the diarrhea foul-smelling?"

"Please rate your abdominal pain on a scale of 1-10, with 10 being the worst pain of your life"

"Are the symptoms worse in the morning or at night?"

A 6-year-old boy is brought to the physician because he has a limp for 3 weeks. He was born at term and has been healthy since. His immunization are up-to-date; he received his 5th DTaP vaccine one month ago. He is at 50th percentile for height and weight. His temperature is 37°C (98.6°F), pulse is 80/min and respirations are 28/min. When asked to stand only on his right leg, the left pelvis sags. The remainder of the examination shows no abnormalities. Which of the following is the most likely cause of these findings?

Damage to the right superior gluteal nerve

Damage to the right common peroneal nerve

Damage to the left inferior gluteal nerve

Sacral dermatomes and myotomes for USMLE Step 3: High-Yield Anatomy Lessons

Sacral Plexus - Nerve Root Rundown

Sacral Plexus: Nerve Roots L4-S4 and Major Branches

Roots: Formed from the anterior rami of spinal nerves L4-S4.
Key Nerves & Roots:
- Superior Gluteal (L4-S1): Innervates gluteus medius, minimus.
- Inferior Gluteal (L5-S2): Innervates gluteus maximus.
- Sciatic (L4-S3): Composed of tibial and common peroneal divisions.
- Pudendal (S2-S4): Innervates perineum. 📌 Superior gluteal, Inferior gluteal, Sciatic, Pudendal.

⭐ The common peroneal nerve is the most frequently injured nerve in the lower limb, often due to its superficial position at the fibular neck, leading to foot drop.

Sacral Dermatomes - The Sensation Map

Dermatome Map: Anterior, Posterior, and Spinal Cord Views

S1: Lateral aspect of the foot, heel, and sole.
S2: Posterior thigh and calf.
S3: Medial aspect of the thigh and perineum.
S4-S5: Perianal region, forming concentric rings.

⭐ Saddle Anesthesia: Numbness in the S2-S5 distribution (buttocks, perineum, inner thighs) is a classic sign of Cauda Equina Syndrome, a surgical emergency.

📌 Mnemonic: Think "S is for Saddle" - the S2-S5 dermatomes cover the area that contacts a horse saddle.

Sacral Myotomes - Motorway to Movement

Sacral Myotome Contributions to Lower Limb Movement

S1: Primarily drives ankle plantarflexion (e.g., standing on tiptoes). Also involved in foot eversion and hip extension.
- Key muscles: Gastrocnemius, soleus.
S2: Governs toe flexion and contributes to ankle plantarflexion.
S3-S4: Innervate intrinsic foot muscles and, critically, the external urethral and anal sphincters for urinary and fecal continence.
- The "anal wink" reflex tests the integrity of S3-S4.
📌 Mnemonic: "S2, S3, S4 keeps the pee and poop off the floor."

⭐ The Achilles tendon reflex (ankle jerk) is the classic clinical test for the integrity of the S1 nerve root.

Clinical Correlations - Sacral Nerve Syndromes

Cauda Equina Syndrome (CES): Medical emergency from compression of lumbar, sacral, and coccygeal nerve roots.
- Etiology: Disc herniation (most common), tumor, trauma, abscess.
- Presentation: Asymmetric, radicular pain; saddle anesthesia (S2-S5); bowel/bladder dysfunction (late finding); LMN signs only (hyporeflexia, flaccid paralysis).
Conus Medullaris Syndrome: Injury to the terminal spinal cord (L1-L2).
- Presentation: Sudden, symmetric symptoms; less severe radicular pain; early bowel/bladder dysfunction; mixed UMN & LMN signs.

Conus Medullaris vs Cauda Equina Compression

⭐ Exam Favorite: Conus Medullaris syndrome often presents with a combination of upper motor neuron (hyperreflexia) and lower motor neuron (fasciculations) signs, as it involves both the spinal cord and nerve roots.

High‑Yield Points - ⚡ Biggest Takeaways\n\n> * S1 radiculopathy classically presents with a lost Achilles reflex and weakness in plantarflexion.\n> * S2-S4 roots control bowel, bladder, and erectile function (parasympathetic).\n> * Saddle anesthesia (perineal numbness) is the key sign of S3-S5 involvement.\n> * Cauda Equina Syndrome is a surgical emergency: look for saddle anesthesia, urinary retention, and fecal incontinence.\n> * The anal wink reflex (S3-S5) is a vital physical exam test for sacral nerve integrity.

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