Enthesitis-related pain: Given the patient's history of Ankylosing Spondylitis (AS), a form of spondyloarthritis, enthesitis (inflammation at the sites where tendons or ligaments insert into the bone) is a common feature. The front of the elbow is a typical location for enthesitis, especially considering the patient's underlying condition.

Bursitis: Prepatellar or olecranon bursitis could cause pain over the front of the elbow, especially if the patient has been engaging in activities that involve repetitive elbow flexion or direct pressure on the elbow.
Tendinopathy (e.g., medial epicondylitis): Although more commonly associated with lateral epicondylitis (tennis elbow), medial epicondylitis (golfer's elbow) could also cause pain in the area, especially if there's been a change in activity levels or types.
Referred pain from the neck or shoulder: Given the patient's history of AS, which can affect the spine and lead to altered biomechanics, pain could be referred from the cervical spine or shoulder region to the elbow area.

Infection (septic arthritis or bursitis): Although less likely, infection in the elbow joint or surrounding bursae is a medical emergency that requires prompt diagnosis and treatment to prevent serious complications.
Fracture or avulsion injury: A fracture or avulsion injury could occur, especially if the patient has experienced a fall or direct trauma to the elbow, and must be ruled out, particularly in the context of possible osteoporosis associated with long-standing AS.
Neurological causes (e.g., ulnar nerve entrapment): Ulnar nerve entrapment at the elbow could cause pain and other neurological symptoms in the forearm and hand, which might be confused with or contribute to elbow pain.

Gout or pseudogout: Although more commonly affecting the lower extremities, gout or pseudogout could theoretically cause elbow pain if the patient has a history of these conditions.
Sarcoidosis or other granulomatous diseases: These could cause musculoskeletal symptoms, including joint pain, but would be less likely without other systemic symptoms or a known history of such conditions.
Tumors (benign or malignant): Tumors affecting the bone, soft tissue, or nerves around the elbow are rare but must be considered in the differential diagnosis, especially if the pain is persistent, worsening, or accompanied by other systemic symptoms like weight loss or fever.