The peel-back mechanism refers to the torsional force that the long head of the biceps brachii places on the glenoid labrum, during shoulder abduction and end-range external rotation. This movement is common in baseball and other overhead sports. While the long head of the triceps brachii, the supraspinatus, and subscapularis are important for overhead shoulder stability, they do not contribute to the peel-back mechanism.

The lateral ligament of the ankle, which connects the fibula to the bones of the foot, is much weaker than the medial/deltoid ligament. For this reason, the lateral ligament is injured much more often in an ankle twist.

The supinator muscle is innervated by the radial nerve and functions to supinate the forearm. The biceps brachii muscle also helps supinate the forearm and is innervated by the musculocutaneous nerve.

The median nerve and ulnar nerve mostly serve to innervate the muscle of the hand and wrist. The axillary nerve innervates the teres minor and deltoid.

The anterior interosseous nerve (AIN) is a motor branch of the median nerve that innervates the flexor pollicus longus, pronator quadratus, and the radial half of flexor digitorum profundus. Injury to the AIN results in weakness of the pincer function of the thumb and index fingers. When asked to make an "OK sign, people with an AIN injury make a triangle instead.

You would expect this patient to exhibit wrist drop.

This is a multi-step thinking question that gives you many clues as to what might be wrong in the patient. First, let's sort out the facts that we are given:

1. The patient has a break in the middle of his right humerus.

2. Patient has numbness on the dorsal surface of his ipsilateral (same side) hand.

3. Patient is unable to extend (straighten) his elbow.

Now let's ask ourselves some questions about these facts:

1. What is the anatomy of the humerus, specifically in the mid-shaft? (hint: what "groove" is located there?)

2. What nerve provides sensation to the dorsum of the hand?

3. What muscles extend the elbow and what nerve innervates those muscles?

The spiral groove is located in the middle of the humerus, which is where the radial nerve wraps around the bone. The radial nerve supplies sensation to the dorsum of the hand. The triceps brachii are responsible for straightening the elbow and are innervated by the radial nerve. Fomr the given information, we know the radial nerve is likely injured, but we need to figure out what else could be impacted by this deficit. The radial nerve innervates the extensor muscles of the forearm, allowing one to extend at the wrist. If these muscles were to be deficient, as exhibited in a radial nerve injury, one would expect the wrist to not be able to extend, and thus manifest as wrist drop.

Let's touch on the other answer choices for further learning:

Claw hand is seen in ulnar nerve injury such as Klumpke's paralysis and manifests as a weakness/inability to flex the wrist (flexor carpi ulnaris), the metacarpophalagneal joints of the 4th and 5th digits in extension (interosseous muscles), and interphalagneal joints of the 4th and 5th digits in flexion (also interossei and lumbricals).

Erb-Duchenne Palsy or Erb's Palsy is an injury to the upper trunk of the brachial plexus that occurs when the head is violently displaced from the shoulder as happens in a difficult breech delivery or trauma.  This will involve the suprascapular, musculocutaneous, and often axillary nerves. Patients present with the arm adducted (deltoid muscle deficient), elbow extended (biceps brachii deficient), and forearm pronated (also biceps brachii). This is referred to as the "waiter's tip" position.

Carpal tunnel syndrome is entrapment of the median nerve in the carpal tunnel (beneath the transverse carpal ligament). Patients present with numbness and tingling in the palm of the hand as well as 1st, 2nd, 3rd, and half of the 4th digits. There may be atrophy of the thenar eminence, as well as weakness in thumb opposition. Symptoms are worse at night.

Anterior interosseous nerve (AIN) syndrome is a rare median nerve entrapment that will manifest similar to carpal tunnel syndrome, but is distinguished by its presentation of only motor symptoms and lack of nighttime symptoms.

The ulnar nerve was compressed at the condylar groove. This common phenomenon is often referred to as the "funny bone". The ulnar nerve provides sensory innervation to half of the 4th digit and the entire 5th digit, as well as the medial aspect of the hand.

The ulnar nerve arises from the lower trunk of the brachial plexus, from spinal roots C8 and T1. It travels medially in the arm in the medial cord and courses posteriorly around the medial epicondyle of the elbow in an indentation known as the condylar groove. This is the spot that you hit when you hit your "funny bone."

Other common places of ulnar nerve entrapment are at the cubital tunnel of the elbow or the ulnar tunnel (Guyon's canal) of the wrist. Compression of the radial nerve by the extensor carpi radialis brevis leads to posterior interosseous nerve (PIN) syndrome. The median nerve is commonly compressed at the bicipital aponeurosis and the pronator teres to give pronator syndrome.

This man is suffering from tardy ulnar nerve palsy as a result of cubitus varus.

We are told that the patient has numbness and weakness in his medial hand and 4th and 5th digits; this area is innervated by branches of the ulnar nerve. Commonly seen in these patients are also atrophy of the hypothenar region, as those muscles are also innervated by the ulnar nerve.

Tardy ulnar nerve palsy is a condition in which the ulnar nerve is irritated as a result of cubitus varus of the distal humerus. This condition develops after a child suffers a supracondylar fracture of the humerus with healing resulting in deformity, especially of the medial epicondyle where the ulnar nerve runs.

Incorrect choices:

Carpal tunnel syndrome is entrapment of the median nerve in the carpal tunnel (beneath the transverse carpal ligament). Patients present with numbness and tingling in the palm of the hand as well as 1st, 2nd, 3rd, and half of the 4th digits. There may be atrophy of the thenar eminence as well as weakness in thumb opposition. Symptoms are worse at night.

Klumpke's paralysis is a result of injury to the lower trunk of the brachial plexus, or the spinal roots C8 and T1. The classic manifestation is the claw hand, characterized by weakness/inability to flex the wrist (flexor carpi ulnaris), the metacarpophalagneal joints of the 4th and 5th digits in extension (interosseous muscles), and interphalagneal joints of the 4th and 5th digits in flexion (also interossei and lumbricals).

Posterior interossesous nerve (PIN) syndrome is compression of a branch of the radial nerve in the radial tunnel resulting in weakened extension of the hand and wrist.

Thoracic outlet syndrome is caused by an abnormal insertion of the anterior and middle scalene muscles and the cervical rib attached to C7 that causes nerve and vascular compression. There are a wide range of symptoms and may cause vascular issues as well.

The knife probably hit the posterior cord of the brachial plexus.

To answer this question we need to think about what deficits are exhibited by the patient:

1. Inability to abduct the shoulder

2. Inability to extend the shoulder

3. Inability to extend the elbow

Let's think about what nerves are deficient:

1. The axillary nerve supplies the deltoid muscle, which function to abduct the shoulder.

2. The latissimus dorsi extends the shoulder and is supplied by the thoracodorsal nerve.

3. Elbow extension is accomplished by the triceps brachii, which is innervated by the radial nerve.

So we know the axillary, thoracodorsal, and radial nerves are deficient. These three nerves all arise from the posterior cord of the brachial plexus, however they also receive innervation from the upper trunk of the brachial plexus. We know that the patient is able to fully flex the shoulder and elbow, which is accomplished by the coracobrachialis and biceps brachii, respectively. These two muscles are both innervated by the musculocutaneous nerve. Because the musculocutaneous nerve is intact, we know the upper trunk of the brachial plexus has not been affected, and that the knife must have pierced the posterior cord.

C8 and T1 supply the lower trunk of the brachial plexus, which gives rise to the ulnar nerve. The anterior divisions supply the medial and lateral cords of the brachial plexus. The median nerve is supplied by the medial and lateral cords of the brachial plexus and innervates the functions of wrist and hand flexion.

Irritation of the phrenic nerve can result in referred pain to the ipsilateral shoulder. In this case, since the abscess irritates the right disphragm, the patient will have pain in his right shoulder.

The phrenic nerve is supplied by C3, C4, and C5 and innervates the diaphragm for breathing purposes (C3, 4, 5 keep the diaphragm alive). Thinking back to sensory innervation of the shoulder, we know that the suprascapular nerve (C3, C4) and axillary nerve (C5, C6) supply the shoulder area. Specifically, the referred pain to the ipsilateral shoulder from phrenic nerve injury/irritation is known as "Kehr's Sign".

This man is suffering from acute inflammatory demyelinating polyradiculoneuropathy (AIDP), also known as Guillain-Barre Syndrome. As suggested by the name, the pathology is due to demyelination.

AIDP is a symmetric ascending paralysis that begins in the feet and ascends to the trunk and upper extremities. The paralysis will often impair breathing and frequently affects the facial nerve as well. This disease commonly follows an illness, especially infectious diarrhea caused by Campylobacter jejuni. The immune response to foreign antigens mistakenly targets host nerve tissues, specifically the myelin sheaths.

Inhibition of ACh release into synaptic cleft can also result in weakness, but describes the effects of botulinum toxin in botulism. The toxin is made by the bacteria Clostridium botulinum as a foodborne illness. Patients experience weakness, trouble seeing, fatigue, and possible speech difficulties. Muscular weakness may follow. Diagnosis is made by identifying the toxin in food or feces.

Blockade of action potentials is seen in conduction blockages as a result of nerve injury, such as neuropraxia, axonotmesis, or neurotmesis.

Blockade of presynaptic voltage-gated calcium channels describes Lambert-Eaton Syndrome, a presynaptic neuromuscular junction disorder characterized by muscular weakness of the limbs. It is an autoimmune condition that attacks presynaptic voltage-gated calcium channels. Proximal muscles are affected first, so patients have difficulty climbing stairs or standing up from sitting. They also may have difficulty reaching over their head or combing their hair. Diagnosis is made by EMG, which shows normal latency and conduction velocities, but small amplitudes. Rapid bursts of stimuli or exercising the muscle will lead to greatly increased amplitudes due to an influx of calcium.

Blockade of postsynaptic ACh receptors is seen in myasthenia gravis, a postsynaptic neuromuscular junction disorder very similar to Lambert-Eaton Syndrome. It is an autoimmune disorder directed against postsynaptic acetylcholine receptors and is characterized by easy fatigue. Muscles become progressively weaker with activity, so patients tend to have symptoms at the end of the day. Earliest signs include difficulty keeping the eyes open, or difficulty with chewing, speaking, or swallowing. EMG studies will show progressively decreasing amplitudes with repeated stimuli.