CSF sampling can be performed from which of the following cisterns?

• A. Cerebellomedullary cistern
• B. Fourth ventricle
• C. Lateral ventricle
• D. Subarachnoid space along the spinal cord

Answer: (A)

CSF taps target dilated areas of the subarachnoid space called cisterns. The cerebellomedullary cistern (cisterna magna) is the largest cranial cistern, located between the cerebellum and medulla, and is accessible via a dorsal suboccipital approach. It provides a reliable CSF sample with minimal risk to brain tissue when performed correctly. The fourth ventricle and lateral ventricles are part of the ventricular system, not cisterns, so sampling there would involve penetrating brain tissue and carries greater risk. The subarachnoid space along the spinal cord is not a cistern; it's used for lumbar taps, not cisternal access. So, the cerebellomedullary cistern is the appropriate site for cisternal CSF sampling.