A healthy 22-year-old male participates in a research study you are leading to compare the properties of skeletal and cardiac muscle. You conduct a 3-phased experiment with the participant. In the first phase, you get him to lift up a 2.3 kg (5 lb) weight off a table with his left hand. In the second phase, you get him to do 20 burpees, taking his heart rate to 150/min. In the third phase, you electrically stimulate his gastrocnemius with a frequency of 50 Hz. You are interested in the tension and electrical activity of specific muscles as follows: Biceps in phase 1, cardiac muscle in phase 2, and gastrocnemius in phase 3. What would you expect to be happening in the phases and the respective muscles of interest?

Increase of tension in all phases

Increase of tension in experiments 2 and 3, with the same underlying mechanism

Recruitment of large motor units followed by small motor units in experiment 1

Fused tetanic contraction at the end of all three experiments

Recruitment of small motor units at the start of experiments 1 and 2

A 25-year-old man comes to the physician for severe back pain. He describes the pain as shooting and stabbing. On a 10-point scale, he rates the pain as a 9 to 10. The pain started after he lifted a heavy box at work; he works at a supermarket and recently switched from being a cashier to a storekeeper. The patient appears to be in severe distress. Vital signs are within normal limits. On physical examination, the spine is nontender without paravertebral muscle spasms. Range of motion is normal. A straight-leg raise test is negative. After the physical examination has been completed, the patient asks for a letter to his employer attesting to his inability to work as a storekeeper. Which of the following is the most appropriate response?

I understand that you are uncomfortable, but the findings do not match the severity of your symptoms. Let's talk about the recent changes at your job.

“Yes. Since work may worsen your condition, I would prefer that you stay home a few days. I will write a letter to your employer to explain the situation.”

You say you are in severe pain. However, the physical examination findings do not suggest a physical problem that can be addressed with medications or surgery. I'd like to meet on a regular basis to see how you're doing.

The physical exam findings do not match your symptoms, which suggests a psychological problem. I would be happy to refer you to a mental health professional.

The physical exam findings suggest a psychological rather than a physical problem. But there is a good chance that we can address it with cognitive-behavioral therapy.

A 51-year-old woman comes to the physician because of a 1-year history of occasional discoloration and tingling in her fingers. She has no history of major medical illness and takes no medications. Examination of the hands and fingers shows thickened, waxy skin and several firm white nodules on the fingertips. Further evaluation of this patient is most likely to show which of the following findings?

Endocardial immune complex deposition

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?"

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

Thoracic Dermatomes - The Body's Segments

Thoracic dermatomes are specific, band-like segments of skin innervated by the sensory fibers of a single thoracic spinal nerve. These bands wrap horizontally around the trunk from the posterior midline to the anterior midline.
Key Anatomical Landmarks:
- T4: Level of the nipples (📌 "T4 at the teet pore").
- T6: Level of the xiphoid process.
- T10: Level of the umbilicus (navel) (📌 "T10 at the belly but-ten").
- T12: Level of the suprapubic region, just above the inguinal ligament.

Clinical Application:
- Essential for localizing the level of a spinal cord lesion or nerve root compression.
- The rash of Herpes Zoster (shingles) characteristically erupts within a single dermatome.
- Visceral referred pain: Cardiac pain can be referred to the T1-T4 dermatomes.

⭐ High-Yield Fact: Due to the upward angulation of spinal nerves as they exit the vertebral column, the sensory level of a spinal cord injury is often found one to two dermatomal levels below the vertebral level of the lesion itself.

Thoracic Myotomes - Core Muscle Control

Primary Function: Innervate intercostal and abdominal wall muscles, crucial for respiration, posture, and core stability.
Upper Thoracic (T1-T6):
- Mainly supply the intercostal muscles.
- Action: Elevate and depress the ribs during breathing.
Lower Thoracic (T7-T12):
- Supply both intercostal and abdominal muscles (rectus abdominis, obliques).
- Action: Forced expiration, coughing, vomiting, and trunk flexion/rotation.

⭐ Clinical Pearl (Beevor's Sign): With a spinal cord lesion at the T10 level, asking a patient to sit up causes the umbilicus to move upwards due to unopposed upper abdominal muscle action.

📌 Mnemonic: "T1-6 breathe, T7-12 squeeze."

Clinical Correlations - Nerves Under Pressure

Thoracic Outlet Syndrome (TOS): Compression of the brachial plexus and/or subclavian vessels.
- Sites of Compression: Scalene triangle, costoclavicular space, sub-pectoralis minor space.
- Neurogenic TOS (>90%): Pain, paresthesia in C8/T1 dermatomes (ulnar side of forearm/hand), and intrinsic hand muscle weakness (myotomes).
- Venous TOS: Upper extremity swelling, cyanosis (Paget-Schroetter syndrome).
- Arterial TOS: Ischemia, claudication, pallor, and ↓ pulse.
Intercostal Nerve Entrapment:
- Causes: Post-thoracotomy pain syndrome, rib fractures, postherpetic neuralgia (shingles).
- Symptoms: Sharp, burning, or radiating pain in a specific dermatomal band around the chest wall.
Radiculopathy: Nerve root compression at the intervertebral foramen (e.g., from a herniated disc or osteophyte) causing dermatomal sensory loss and myotomal weakness.

⭐ T4 Dermatome: A key landmark for sensory level testing. Loss of sensation below the nipple line can indicate a spinal cord lesion at or above the T4 level.

Thoracic Outlet Syndrome Compression Sites

High-Yield Points - ⚡ Biggest Takeaways

T4 dermatome is at the nipple line.

T10 dermatome is at the umbilicus (bellybuTTon).

Thoracic spinal nerves primarily supply the intercostal muscles, crucial for respiration.

T1 is unique as it contributes to the brachial plexus, innervating intrinsic hand muscles.

Herpes zoster (shingles) presents as a painful, unilateral vesicular rash confined to a single dermatome.

Cardiac pain can be referred to the T1-T4 dermatomes on the left side.

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