Deep Vein Thrombosis (DVT): This is the most likely diagnosis due to the recent surgery, which increases the risk of blood clots. Immobility after surgery and the potential for damage to veins during the procedure further elevate this risk. The symptoms of foot swelling and intense pain are consistent with DVT.

Nerve Injury or Compression: During knee arthroscopy, nerves can be injured or compressed, leading to pain and swelling in the foot. This could be due to the positioning during surgery or direct trauma from the surgical instruments.
Compartment Syndrome: Although less common after arthroscopy, compartment syndrome is a possibility, especially if there was significant bleeding or fluid extravasation into the compartments of the leg during or after the surgery.
Infection: Post-surgical infection could lead to swelling and pain, although it might be expected to involve the surgical site more directly. However, infections can spread, and the presentation might vary.

Pulmonary Embolism (PE): While the symptoms are localized to the foot, it's crucial not to miss the possibility of a pulmonary embolism, especially if the patient has risk factors or symptoms suggestive of PE, such as shortness of breath or chest pain. A DVT can break loose and travel to the lungs, which is life-threatening.
Septic Arthritis or Osteomyelitis: Infection within the joint or bone can be devastating and requires prompt treatment. Although the primary complaint is foot swelling, the recent surgery increases the risk of infection spreading to the joint or bone.

Complex Regional Pain Syndrome (CRPS): This condition can occur after trauma or surgery and is characterized by intense burning or aching pain in the affected limb. It's less common but should be considered if other diagnoses are ruled out.
Arterial Injury or Thrombosis: Direct injury to arteries during surgery or thrombosis due to the procedure is rare but can lead to severe ischemia and requires immediate attention.