Femoroacetabular Impingement (FAI): This condition is characterized by abnormal contact between the femoral head and the acetabulum, leading to pain and limited mobility, especially with activity. The onset of symptoms over a year ago and the nature of the pain (worse with activity) align with FAI, despite normal X-rays, as early stages or certain types might not show clear radiographic abnormalities.

Trochanteric Bursitis: Inflammation of the bursa over the greater trochanter can cause lateral thigh pain and is often exacerbated by activity. Normal X-rays do not rule out this diagnosis.
Piriformis Syndrome: This condition involves the piriformis muscle compressing the sciatic nerve, leading to pain in the buttocks and radiating down the thigh. Activity can worsen symptoms, and X-rays would typically be normal.
Stress Fractures: Although X-rays are normal, stress fractures might not be visible on initial radiographs, especially if they are hairline or in early stages. Activity-related pain is a key feature.

Avascular Necrosis (AVN): This condition involves the death of bone tissue due to a lack of blood supply. It can present with hip pain and might not be immediately visible on X-rays. Missing AVN could lead to significant morbidity.
Slipped Capital Femoral Epiphysis (SCFE): Although more common in younger patients, SCFE can occur in young adults. It's a serious condition that requires prompt diagnosis and treatment to avoid long-term damage.
Infection (Osteomyelitis or Septic Arthritis): Infections in the bone or joint can present with pain and might not have specific findings on X-rays in early stages. These conditions are medical emergencies.

Synovial Chondromatosis: A rare condition characterized by the growth of cartilage in the synovium of the joint, leading to pain and mechanical symptoms.
Bone Tumors: Primary bone tumors (benign or malignant) can cause hip pain. While rare, they are important to consider in the differential diagnosis, especially if other causes are ruled out.
Inflammatory Arthropathies (e.g., Rheumatoid Arthritis, Ankylosing Spondylitis): These conditions can cause hip pain and might have a more systemic presentation. They are less common in this age group but should be considered, especially if there are other symptoms or findings suggestive of an inflammatory process.