Acute low back pain radiating to the anterior thigh and causing weakness in ankle dorsiflexion is likely due to a herniated disc at which spinal level?

The presentation of acute low back pain radiating to the anterior thigh, along with weakness in ankle dorsiflexion, strongly suggests involvement of the L4 spinal nerve root. The L4 nerve root exits the spine at the L4-L5 level, and a herniated disc at this location can affect the L4 nerve root directly.

In clinical terms, the L4 nerve root is responsible for sensation over the anterior thigh and can also affect the quadriceps muscle, which is crucial for knee extension. Additionally, while the L4 nerve root participates in ankle dorsiflexion, the weakness in dorsiflexion can also be influenced by adjacent nerve root involvement, particularly with L5.

Understanding the specific dermatome and myotome patterns, as well as potential knee reflex changes, helps in diagnosing the exact level of nerve root involvement. In this case, the combination of symptoms points to L4 as the most likely level affected by a herniated disc.