Deep Vein Thrombosis (DVT): The elevated D-dimer level (956) is a strong indicator of DVT, especially when combined with acute onset of thigh pain. DVT is a common cause of leg pain and swelling, and the D-dimer test is often used as a screening tool for this condition.

Muscle Strain: Acute onset of thigh pain could be due to a muscle strain, which is a common injury, especially in individuals who engage in physical activities. However, the elevated D-dimer level is not typically associated with muscle strain.
Trauma: Recent trauma to the thigh could cause pain and may also lead to a false-positive D-dimer result due to tissue damage. The history of the patient would be crucial in determining the likelihood of this diagnosis.

Pulmonary Embolism (PE): Although the primary symptom is thigh pain, a significantly elevated D-dimer level should prompt consideration of PE, especially if there are any respiratory symptoms or risk factors for PE. Missing a PE could be fatal.
Aortic Dissection: This is a rare but life-threatening condition that could present with leg pain if the dissection involves the iliac or femoral arteries. An elevated D-dimer can be seen in aortic dissection, making it a "do not miss" diagnosis.

Osteonecrosis of the Femur: This condition could cause thigh pain but would not typically cause an elevated D-dimer level. It's more common in specific populations, such as long-term steroid users or heavy alcohol consumers.
Bone Tumor: A bone tumor could cause thigh pain, but it would be unusual for it to cause a significantly elevated D-dimer level unless there was associated thrombosis or tumor necrosis. Bone tumors are relatively rare and would typically have a more gradual onset of symptoms.