If a patient has weakness of quadriceps extension and diminished knee reflex, which nerve root is compressed?

If a patient has weakness of quadriceps extension and diminished knee reflex, which nerve root is compressed?

• A. L3
• B. L5
• C. S1
• D. L4

Answer: (D)

The knee-jerk reflex and the strength of the quadriceps mainly map to the L4 nerve root. The quadriceps muscle, responsible for extending the knee, receives its major innervation from the femoral nerve branches that originate in L2–L4, with L4 contributing most strongly. The patellar (knee-jerk) reflex is a reflex arc that tests the L4 segment. When the L4 nerve root is compressed, you typically see weakness in knee extension and a diminished patellar reflex. Other roots don’t fit this exact pattern as neatly: L3 can contribute to knee extension but is less definitive for the reflex, while L5 and S1 control different movements and reflexes (foot dorsiflexion/toe movements for L5; plantarflexion/Achilles reflex for S1). So the combination described most consistent with L4 compression.

The knee-jerk reflex and the strength of the quadriceps mainly map to the L4 nerve root. The quadriceps muscle, responsible for extending the knee, receives its major innervation from the femoral nerve branches that originate in L2–L4, with L4 contributing most strongly. The patellar (knee-jerk) reflex is a reflex arc that tests the L4 segment. When the L4 nerve root is compressed, you typically see weakness in knee extension and a diminished patellar reflex.

Other roots don’t fit this exact pattern as neatly: L3 can contribute to knee extension but is less definitive for the reflex, while L5 and S1 control different movements and reflexes (foot dorsiflexion/toe movements for L5; plantarflexion/Achilles reflex for S1). So the combination described most consistent with L4 compression.