What reflex is tested at the L4 segment?

The reflex tested at the L4 segment is related to the quadriceps muscle, specifically through the patellar (knee-jerk) reflex. The quadriceps tendon reflex is mediated by the L3-L4 segments of the spinal cord, making it an essential component in evaluating the integrity of the L4 nerve root.

When the patellar tendon is tapped, it causes the quadriceps muscle to contract, resulting in slight extension of the knee. This reflex pathway is an important clinical examination tool because it helps in assessing the neurological function of the lower limb, specifically indicating the health of the L4 segment and its associated structures.

In the context of the provided options, the iliopsoas does have a significant role in hip flexion but is more influenced by the L2-L3 segments. The semitendinosus is primarily associated with nerves from the L5-S1 segments, while the gastrocnemius reflex tests are primarily linked to the S1-S2 segments. Therefore, the correct answer aligns with the reflex that is testing the integrity of the L4 segment specifically through the knee-jerk reaction of the quadriceps muscle.