Step 1: Septic (acute suppurative) arthritis is a pyogenic bacterial infection of a joint, usually reaching the joint by haematogenous spread from a distant site.
Step 2: It is more common in children, who present toxic with fever, severe pain, swelling and redness, with the joint held in the position of ease.
Step 3: Among all joints, the knee is the most commonly affected joint.
Step 4: The hip, shoulder and elbow are also involved but less frequently than the knee, so the answer is knee.