Final answer:
The tennis player's medial thigh pain is likely due to a muscle strain or tear in the muscles of the medial compartment of the thigh, commonly associated with overuse or sudden movements in sports.
Step-by-step explanation:
The probable cause of the tennis player's medial thigh pain, given the symptoms of pain with resisted hip adduction and hip flexion, along with diffuse pain and ecchymosis, is likely a muscle strain or tear in one of the muscles of the medial compartment of the thigh.
These muscles, which include the adductor longus, adductor brevis, adductor magnus, and pectineus, are responsible for adducting the femur at the hip. Overuse or a sudden movement can cause microtrauma leading to an inflammatory response and tendinitis, which is consistent with the presented symptoms.
Treatment typically requires rest, avoidance of activity that aggravates the condition, and a gradual return to activity with proper strengthening exercises to prevent reoccurrence.