Which reflex is tested with the patellar tendon?

The reflex that is tested with the patellar tendon is indeed associated with the L4 nerve root. When the patellar tendon is tapped, it triggers a reflex arc that involves the quadriceps muscle and results in extension of the knee. This reflex is part of the classic knee-jerk reflex.

The L4 nerve root is responsible for innervating the quadriceps muscle, which is crucial for the response observed during the test. When this reflex is elicited, it indicates that the pathway involving the L4 spinal segment, spinal cord, and appropriate nerves is intact and functioning correctly. This is why the answer is considered correct; the patellar reflex checks the integrity of not just the lower motor neuron pathways but also the specific spinal level L4 that correlates with this reflex action.