Final answer:
The axillary nerve is likely injured if a patient cannot abduct and laterally rotate their arm due to damage to the deltoid and teres minor muscles. This nerve innervates these muscles and is tested by resistance exercises during physical examination.
Step-by-step explanation:
The axillary nerve is likely injured if a patient has damage to the deltoid and teres minor muscles and is unable to abduct and laterally rotate their arm. The deltoid is the major abductor of the arm, while the teres minor laterally rotates and extends the arm. These muscles are innervated by the axillary nerve, which originates from the posterior cord of the brachial plexus.
In clinical examination, muscle strength is assessed by having the patient contract muscles against resistance. For example, to test the deltoid muscle, the patient might be asked to lift the arm against resistance provided by the examiner. Similarly, to assess the teres minor, one might test the arm's lateral rotation against resistance. If there is a noticeable difference in the ability to perform these actions on one side compared to the other, it may suggest nerve damage on the weaker side.
Diseases that affect the motor neurons can also be evaluated in this manner. Muscle tone without voluntary movement can indicate an upper motor neuron lesion, whereas muscles without tone may suggest a lower motor neuron lesion.