Drug not affecting CSF synthesis & absorption

Drug not affecting CSF synthesis & absorption and also does not affect ICP is:-

A. Ketamine
B. Thiopentone sodium
C. Nitrous oxide
D. Halothane

Ans. C. Nitrous Oxide.

 Nitrous Oxide.

- It increases cerebral blood flow and volume hence producing
- a mild elevation of intracranial pressure
Thiopentone sodium
- It causes vasoconstriction of CNS blood vessels,
- Thus decreasing cerebral blood volume and intracranial pressure ICP. 
Ketamine
- Increases the cerebral blood flow and intracranial pressure
- Thus, ketamine is relatively contraindicated in patients with an intracranial mass or increased ICP
Halothane
- It causes dilation of cerebral vessels, thus
- Decreases cerebral vascular resistance and increases cerebral blood flow
- Hence, the intracranial pressure rises
Nitrous Oxide does not affect CSF dynamics and ICP as compared to the other anesthetic drugs in the question. Hence it is the best possible answer.