Rotator Cuff Tendinitis or Tear: This condition is the most likely cause of shoulder pain with external rotation, especially when there's difficulty in actively raising the arm above the head or abducting the arm without assistance. The rotator cuff muscles and tendons play a crucial role in shoulder movements, and their injury can lead to significant impairment in these actions.

Adhesive Capsulitis (Frozen Shoulder): Characterized by pain and stiffness in the shoulder, adhesive capsulitis can lead to limited range of motion, including difficulty with abduction and external rotation. The condition often progresses through stages, starting with pain, followed by stiffness, and then possibly recovery.
Subacromial Bursitis: Inflammation of the subacromial bursa can cause pain, especially with movements like abduction and external rotation. This condition can coexist with rotator cuff tendinitis or tears.
Labral Tears: Tears in the labrum, a cartilage structure surrounding the socket of the shoulder joint, can cause pain and mechanical symptoms, including catching or locking, especially with movements involving external rotation and abduction.

Proximal Humerus Fracture: Although less common, a fracture of the proximal humerus can present with severe pain and limited mobility of the shoulder, including difficulty with external rotation and abduction. This diagnosis is critical to identify due to the potential need for surgical intervention.
Axillary Nerve Injury: Damage to the axillary nerve can result in weakness of the deltoid muscle and teres minor, leading to difficulties with shoulder abduction and external rotation. Identifying nerve injuries is crucial for appropriate management and rehabilitation.
Septic Arthritis or Osteomyelitis: Infection within the shoulder joint (septic arthritis) or bone (osteomyelitis) is a medical emergency requiring prompt diagnosis and treatment. Symptoms can include severe pain, limited mobility, and systemic signs of infection.

Quadrilateral Space Syndrome: Compression of the axillary nerve and posterior circumflex humeral artery within the quadrilateral space can lead to shoulder pain and weakness, particularly with abduction and external rotation.
Parsonage-Turner Syndrome (Neuralgic Amyotrophy): A rare condition characterized by acute, severe pain in the shoulder and arm, followed by patchy muscle weakness and atrophy. It can mimic other shoulder conditions but has a distinct clinical course and requires specific management.