A patient presents with long time history of knee pain suggestive of osteoarthritis. Now he complains of unilateral lower limb swelling and on examination there is +ve pedal & tibial pitting edema

A patient presents with long time history of knee pain suggestive of osteoarthritis. Now he complains of unilateral lower limb swelling and on examination there is +ve pedal & tibial pitting edema. What is the next appropriate investigation?
a. CXR
b. ECG
c. Echocardiography
d. Duplex ultrasound of lower limb (immobility can lead to DVT).