A knee-jerk test can be used to evaluate which system?

The knee-jerk test, also known as the patellar reflex test, is primarily used to evaluate the functioning of the nervous system. This test assesses the integrity of the reflex arc, which involves sensory and motor pathways that are part of the nervous system. When the patellar tendon is tapped, it causes a reflex action resulting in the contraction of the quadriceps muscle, which is an involuntary response.

This reflex is a simple pathway that helps to reveal the health of the nervous system, particularly the spinal cord and various nerve pathways involved in reflex actions. Thus, the knee-jerk test is a crucial tool in diagnosing potential issues within the nervous system, distinguishing it from the other systems mentioned, such as digestive, cardiovascular, and respiratory. These systems are evaluated through different means specific to their functions and pathologies.