Trochanteric Bursitis: This condition is characterized by inflammation of the bursa located at the outer aspect of the thigh, near the hip. Climbing and walking can exacerbate the pain, and it often develops without a specific injury. The symptoms and activities that aggravate the pain align closely with trochanteric bursitis.

Iliotibial (IT) Band Syndrome: This condition involves inflammation of the iliotibial band, a ligament that runs down the outside of the thigh from the hip to the knee. Activities like climbing and walking can irritate the IT band, causing pain. The lack of a specific injury and the nature of the activities that worsen the pain make IT band syndrome a plausible diagnosis.
Piriformis Syndrome: This condition occurs when the piriformis muscle, located in the buttock region, irritates or compresses the sciatic nerve. Pain can be felt in the thigh and is often exacerbated by activities that involve climbing or walking. While it might not be the first consideration, piriformis syndrome is a possible cause given the symptoms.

Atherosclerotic Peripheral Artery Disease (PAD): Although less common in younger individuals, PAD can cause leg pain during walking or climbing due to reduced blood flow. It's crucial to consider this diagnosis because untreated PAD can lead to severe complications, including limb loss.
Stress Fracture: Despite the absence of a reported injury, stress fractures can occur from repetitive stress without a single traumatic event. They are a critical diagnosis to consider because they can lead to complete fractures if not properly managed.
Osteonecrosis of the Femoral Head: This condition involves the death of bone tissue due to a lack of blood supply. It can cause hip and thigh pain, especially with weight-bearing activities. Early diagnosis is vital to prevent collapse of the femoral head and subsequent severe arthritis.

Neoplastic Processes (e.g., Osteosarcoma, Soft Tissue Sarcomas): Although rare, tumors can cause thigh pain. These conditions are typically accompanied by other symptoms such as weight loss, night pain, or a palpable mass.
Inflammatory Arthropathies (e.g., Rheumatoid Arthritis, Ankylosing Spondylitis): These conditions can cause thigh pain among other systemic symptoms. They are less likely given the localized nature of the pain but should be considered, especially if other symptoms are present.
Chronic Compartment Syndrome: This condition involves increased pressure within a muscle compartment, leading to pain during exercise. It's a rare cause of thigh pain but should be considered in athletes or individuals with high levels of physical activity.