Cervical Radiculopathy: This condition, often due to a herniated disc or foraminal stenosis, compresses or irritates a cervical nerve root, leading to pain, numbness, and sometimes weakness in the arm. The symptoms described are classic for this condition, making it the most likely diagnosis.

Cervical Spondylosis: This is a common condition in older adults characterized by wear and tear on the discs and joints in the neck, which can cause pain and numbness radiating down the arm.
Thoracic Outlet Syndrome: Compression of the nerves and/or blood vessels that pass into the arm through the thoracic outlet (the space between your collarbone and first rib) can cause pain, numbness, and tingling in the arm.
Brachial Plexus Injury: Trauma or compression of the brachial plexus, a network of nerves that conducts signals from the spinal cord to the shoulder, arm, and hand, can lead to pain, numbness, and weakness in the arm.

Cervical Spinal Cord Compression: Conditions such as spinal stenosis, tumors, or abscesses can compress the spinal cord, leading to severe and potentially irreversible neurological deficits if not promptly addressed.
Acute Coronary Syndrome: Although less common, pain radiating down the left arm can be a symptom of a heart attack, especially in women, and is crucial not to miss due to its high mortality rate.
Pulmonary Embolism: In rare cases, a pulmonary embolism can cause referred pain in the arm, and given its high mortality rate, it should always be considered.

Pancoast Tumor: A type of lung cancer located at the top of the lung that can invade the brachial plexus, causing arm pain and numbness.
Multiple Sclerosis: An autoimmune disease that affects the brain and spinal cord, potentially causing a wide range of neurological symptoms, including numbness and pain in the arms.
Vascular Malformations or Aneurysms: Abnormal formations of blood vessels in the neck or arm can cause neurological symptoms due to compression or thromboembolic events.