Final answer:
The most useful treatment for intractable post-lumbar puncture headache is a repeat lumbar puncture with blood patch.
Step-by-step explanation:
The most useful treatment for intractable post-lumbar puncture headache (PLPH) is repeat lumbar puncture with blood patch (option A). PLPH is a common complication of lumbar puncture, caused by leakage of cerebrospinal fluid from the puncture site, which leads to low pressure in the spinal canal. A blood patch involves injecting a small amount of the patient's own blood into the epidural space to seal the puncture site and restore normal pressure.