USMLE Musculoskeletal Anatomy Practice Questions with Answers

Concept Explanation

USMLE musculoskeletal anatomy is the study of the bones, muscles, nerves, and vasculature of the human body as they relate to clinical pathology and physical examination findings on the medical licensing exams.

This subject demands a high level of precision, particularly regarding the innervation and blood supply of the limbs. For instance, understanding the brachial plexus is essential for diagnosing peripheral nerve injuries. The USMLE focuses heavily on "high-yield" clinical correlations, such as identifying the specific nerve damaged during a humerus fracture or the muscle group affected by a particular spinal root compression. Students must be able to translate anatomical facts into diagnostic reasoning during their USMLE Prep. Key areas of focus include the rotator cuff, the carpal tunnel, the femoral triangle, and the major joints (shoulder, hip, and knee).

Solved Examples

1. Radial Nerve Injury: A patient presents with a midshaft humerus fracture and is unable to extend their wrist. Which nerve is most likely affected?
   1. Identify the anatomical location: The midshaft of the humerus is where the radial nerve travels in the radial groove.
   2. Recall the nerve function: The radial nerve innervates the extensors of the wrist and fingers.
   3. Conclusion: A midshaft humerus fracture frequently results in radial nerve palsy, leading to "wrist drop."
2. Rotator Cuff Muscles: Which muscle of the rotator cuff is responsible for the first 15 degrees of arm abduction?
   1. List the SITS muscles: Supraspinatus, Infraspinatus, Teres minor, and Subscapularis.
   2. Identify functions: The supraspinatus initiates abduction ($0^\circ - 15^\circ$).
   3. Compare with others: The deltoid takes over for abduction from $15^\circ - 90^\circ$.
   4. Conclusion: The supraspinatus is the correct muscle.
3. Lower Limb Vasculature: A surgeon is performing a procedure in the femoral triangle. What is the lateral-to-medial arrangement of the structures?
   1. Use the mnemonic NAVEL: Nerve, Artery, Vein, Empty space, Lymphatics.
   2. Apply the direction: Lateral is "N" (Femoral Nerve) and Medial is "L" (Lymphatics).
   3. Conclusion: From lateral to medial, the order is Femoral Nerve, Femoral Artery, Femoral Vein.

Practice Questions

1. A 24-year-old athlete presents with shoulder pain after a fall. Physical exam reveals difficulty initiating abduction of the arm. An MRI shows a tendon tear. Which nerve innervates the most likely damaged muscle?

2. During a carpal tunnel release surgery, a surgeon accidentally transects a nerve that provides motor innervation to the thenar eminence but does not affect sensation on the palm. Which specific branch was cut?

3. A patient with a fracture of the surgical neck of the humerus is likely to experience weakness in which specific movement of the shoulder?

4. Following a stab wound to the posterior thigh, a patient is unable to flex their knee. Which nerve, derived from the $L4-S3$ roots, is likely damaged?

5. An elderly woman falls and sustains a femoral neck fracture. She is at high risk for avascular necrosis of the femoral head. Which artery provides the primary blood supply that is now compromised?

6. A patient presents with "winging" of the scapula when pushing against a wall. This is caused by paralysis of the serratus anterior muscle. Which nerve is injured?

7. A 45-year-old male presents with numbness in the medial one and a half fingers and weakness in adduction of the thumb. Which nerve is compressed, and at what anatomical site is this most common?

8. Injury to the common peroneal (fibular) nerve at the neck of the fibula typically results in which clinical presentation?

9. A patient is unable to stand on their tiptoes (plantarflexion). Which nerve is most likely affected?

10. Which muscle is the primary flexor of the hip and is innervated by the femoral nerve ($L2-L4$)?

Answers & Explanations

1. Suprascapular nerve: The supraspinatus muscle initiates the first $15^\circ$ of abduction. It is innervated by the suprascapular nerve.
2. Recurrent branch of the median nerve: This nerve supplies the thenar muscles (Opponens pollicis, Abductor pollicis brevis, Flexor pollicis brevis) and is often called the "million-dollar nerve" because its injury causes significant disability.
3. Abduction and External Rotation: A fracture of the surgical neck of the humerus damages the axillary nerve, which innervates the deltoid and teres minor.
4. Sciatic nerve: The sciatic nerve innervates the hamstring muscles (semitendinosus, semimembranosus, and biceps femoris), which are responsible for knee flexion.
5. Medial circumflex femoral artery: This is the main contributor to the blood supply of the femoral head; damage leads to avascular necrosis.
6. Long thoracic nerve: The long thoracic nerve ($C5, C6, C7$) innervates the serratus anterior, which holds the scapula against the thoracic wall.
7. Ulnar nerve at the Guyon canal or Cubital tunnel: The ulnar nerve provides sensation to the medial 1.5 fingers and motor control to the adductor pollicis.
8. Foot drop: The common peroneal nerve divides into the deep and superficial peroneal nerves. Damage leads to loss of dorsiflexion (foot drop) and eversion.
9. Tibial nerve: The tibial nerve innervates the gastrocnemius and soleus muscles, which are the primary plantarflexors of the foot.
10. Iliopsoas: The iliopsoas (psoas major and iliacus) is the strongest flexor of the hip joint.

1. Which nerve is most likely damaged in a patient who presents with a "claw hand" deformity affecting the 4th and 5th digits?

• AMedian nerve
• BRadial nerve
• CUlnar nerve
• DMusculocutaneous nerve

Frequently Asked Questions

What is the most commonly injured rotator cuff muscle?

The supraspinatus is the most frequently injured rotator cuff muscle because of its location beneath the acromion, making it prone to impingement and tears. It is responsible for the initiation of arm abduction.

What nerve is associated with a midshaft humerus fracture?

The radial nerve is the most common nerve injured in midshaft humerus fractures because it runs directly in the radial groove of the bone. This injury typically results in wrist drop due to loss of extensor muscle function.

How does a Trendelenburg gait occur?

Trendelenburg gait occurs when the superior gluteal nerve is damaged, weakening the gluteus medius and minimus muscles. This causes the pelvis to tilt toward the unaffected side when the patient stands on the affected leg.

Which nerve is compressed in carpal tunnel syndrome?

The median nerve is compressed within the carpal tunnel, leading to paresthesia in the first three and a half digits and potential thenar atrophy. You can use an AI Question Generator to practice more specific clinical scenarios involving this condition.

What are the contents of the popliteal fossa?

The popliteal fossa contains the popliteal artery, popliteal vein, tibial nerve, and common fibular nerve. The artery is the deepest structure and is susceptible to damage in posterior knee dislocations.