Final answer:
The likely cause of a postspinal headache after a subarachnoid block is leakage of cerebrospinal fluid (CSF) from the site of the puncture.
Step-by-step explanation:
The likely cause of a postspinal headache after a subarachnoid block is leakage of cerebrospinal fluid (CSF) from the site of the puncture. When a subarachnoid block is performed, a needle is inserted into the spinal canal to deliver medication, such as anesthesia, directly into the cerebrospinal fluid. If the needle punctures the dura mater, the protective membrane surrounding the spinal cord, it can result in a small tear or hole called a dural puncture. This can lead to CSF leakage, which can cause a postspinal headache.