Cervical Radiculopathy: This condition, often resulting from a herniated disk or bone spur in the cervical spine, can compress or irritate nerves, leading to pain in the shoulder and arm, along with numbness in specific fingers, depending on which nerve root is affected. The distribution of pain and numbness to the left arm and index finger suggests involvement of the C6 or C7 nerve root.

Thoracic Outlet Syndrome (TOS): This condition involves compression, injury, or irritation of the nerves and/or blood vessels (arteries and veins) in the thoracic outlet area, which lies between your collarbone (clavicle) and first rib. Symptoms can include pain in your shoulders and neck and numbness in your fingers.
Rotator Cuff Tendinitis or Tear: While primarily causing shoulder pain, severe cases can radiate pain down the arm, especially if there is associated bursitis or if the tear affects the supraspinatus tendon, which could potentially irritate nearby nerves.
Brachial Plexus Injury: An injury to the brachial plexus, the network of nerves that conducts signals from the spinal cord to the shoulder, arm, and hand, can cause pain, numbness, and weakness in the arm.

Myocardial Infarction (MI): Although less common, myocardial infarction can present with referred pain to the left arm. It's crucial to consider cardiac causes, especially in patients with risk factors for coronary artery disease.
Pancoast Tumor: A type of lung cancer located at the top of the lung that can invade nearby structures, including nerves, leading to shoulder and arm pain, along with numbness or weakness.
Stroke or Transient Ischemic Attack (TIA): While typically presenting with sudden onset of neurological deficits, in some cases, pain can be a preceding or accompanying symptom.

Multiple Sclerosis: An autoimmune disease that affects the brain and spinal cord, leading to a wide range of potential neurological symptoms, including numbness and pain in various parts of the body, including the arm.
Peripheral Neuropathy: Conditions that result from damage to your peripheral nerves, which can cause numbness, weakness, and pain, usually in your hands and feet, but can also affect the arms.
Cervical Spinal Stenosis: Narrowing of the spinal canal in the neck, which can put pressure on the spinal cord and the nerves that travel through the spine, potentially causing pain, numbness, and weakness in the neck, shoulders, arms, and hands.