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Cerebrospinal fluid (CSFCerebrospinal fluid, the fluid that cushions the brain and spinal cord.Read more →) matters because a blockage anywhere along its normal flow path causes a build-up of pressure, called hydrocephalus. The exact blockage site changes how it is treated. CSF is a clear fluid produced by the choroid plexus (a fluid-producing tissue) inside the brain's ventricles (fluid-filled cavities), which cushions and nourishes the brain. For example, imagine a tumour blocking one narrow channel between two ventricles: CSF backs up above the blockage, raising pressure, while the system below stays normal. In practice, distinguishing where the blockage sits, inside the ventricles or after CSF leaves them, decides whether surgery can bypass it.

CSFCerebrospinal fluid, the fluid that cushions the brain and spinal cord.Read more → is produced by the choroid plexus within the lateral, third, and fourth ventricles. It flows through the ventricular system, then out into the subarachnoid space (the space surrounding the brain and spinal cord), before being absorbed back into the bloodstream at the arachnoid granulations (small projections on the brain's surface). Obstructive hydrocephalus occurs when a physical blockage inside the ventricular system, such as a tumour, prevents CSF from flowing onward. Communicating hydrocephalus occurs when CSF flows freely out of the ventricles but is then poorly absorbed at the arachnoid granulations, so pressure still builds despite no internal blockage. Say a patient after a subarachnoid haemorrhage (bleeding around the brain) develops gradually worsening confusion and unsteady gait weeks later: blood products clogging the arachnoid granulations are a recognised cause. In practice, obstructive hydrocephalus often needs a procedure to bypass the blockage, whereas communicating hydrocephalus is more often managed with a shunt that drains CSF elsewhere in the body.

• A student asks how the total volume of cerebrospinal fluid compares with the amount produced each day. Which description is most accurate?
• A radiologist distinguishes hydrocephalus caused by blockage within the ventricles from that caused by failed reabsorption outside them. What is the second type called?
• A narrow channel running through the midbrain connects the third and fourth ventricles and is a common site of congenital blockage. What is this channel called?
• A lumbar puncture in a feverish confused man returns cloudy fluid. Which combination of results most strongly supports a bacterial rather than viral cause?

• Obstructive hydrocephalus, a physical blockage within the ventricular system, unlike communicating hydrocephalus's normal internal flow.
• Communicating hydrocephalus, impaired absorption at the arachnoid granulations, rather than a blockage inside the ventricles.
• Choroid plexus, produces CSFCerebrospinal fluid, the fluid that cushions the brain and spinal cord.Read more →, whereas arachnoid granulations absorb it back into the bloodstream.
• Subarachnoid haemorrhage, a recognised cause of communicating hydrocephalus, unlike a tumour which more typically causes obstructive hydrocephalus.

• That term describes a stabilised process rather than this mechanism.
• That opening connects the lateral and third chambers instead.
• Turnover is far faster than once a week.
• The standing volume is about a tenth of this figure.
• The standing volume is roughly ten times this figure.

• Localise the clue to the relevant anatomical level, cell type, pathway or molecular process.
• Map that structure or process to its normal function.
• Predict the deficit or clinical sign produced when it is disrupted.
• Reject options that belong to a neighbouring structure, pathway or level of explanation.