According to Monro-Kellie doctrine, what is the primary compensatory change when intracranial volume increases?

• A. Decrease in brain tissue volume
• B. Reduction of CSF volume
• C. Increase in skull diameter
• D. Increased cerebral blood flow

Answer: (B)

The moving part here is that the skull has a fixed, incompressible volume, so the total intracranial content—brain tissue, CSF, and blood—must stay nearly constant. When intracranial volume increases, the body first compensates by reducing CSF volume because CSF can be displaced or absorbed more readily than the solid brain tissue. This CSF reduction helps offset the added volume and keeps intracranial pressure from rising quickly. If CSF loss isn’t enough to balance the change, venous blood can also be reduced, but the primary and fastest adjustment is a decrease in CSF. The other options don’t fit: increasing brain tissue volume would worsen pressure, the skull diameter can’t expand acutely in adults, and increasing cerebral blood flow would add to the intracranial volume rather than offset it.