B

Cadaveric and imaging studies of thoracic paravertebral block show that injectate frequently reaches the epidural space (≈40 %) and adjacent intercostal spaces, and may extend cranio-caudally within the paravertebral compartments, but it very rarely crosses the dura to enter the subarachnoid (intrathecal) space unless the needle itself punctures the dura [Cowie 2010, PMID 20435949; Seidel 2020, PMID 31678963]. Therefore, diffusion into the subarachnoid space is the least likely.