Final answer:
Isolated atrophy of the infraspinatus muscle is caused by entrapment of the suprascapular nerve at the spinoglenoid notch.
Step-by-step explanation:
Entrapment of the suprascapular nerve at the spinoglenoid notch causes isolated atrophy of the infraspinatus muscle. The suprascapular nerve passes through the suprascapular notch and gives off a branch to the supraspinatus muscle before continuing to the spinoglenoid notch. It is at this notch that the nerve can become entrapped, leading to the isolated atrophy of the infraspinatus muscle, which is located inferior to the spine of the scapula and is responsible for lateral rotation of the shoulder joint.