Rut's answer is correct. If the ball hits the arm and then bounces off the defender's leg accidentally, no violation. If the ball hits the arm and the defender then intentionally strikes the ball with his/her leg, you have a violation. As the official, you have to judge if it was intentional or not.
You noted the "lower leg" in your last sentence which is another good point. If it hits the defenders leg above the knee, it would not be a violation regardless of whether or not it was intentional.
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