**Core Concept**
Intracranial pressure (ICP) is the pressure exerted by the brain and its contents within the cranial vault. Normal ICP is critical for maintaining cerebral perfusion and preventing brain damage. The Monro-Kellie doctrine explains that the sum of the volumes of the brain, blood, and cerebrospinal fluid (CSF) within the cranial vault remains relatively constant.

**Why the Correct Answer is Right**
Normal ICP is approximately 10-15 mmHg (millimeters of mercury) in adults, with a slight increase in pressure during sleep and a slight decrease during wakefulness. The pressure is measured using techniques such as lumbar puncture or intracranial pressure monitoring devices. The ICP is influenced by the balance between the volume of CSF produced and reabsorbed, as well as the blood volume within the cranial vault.

**Why Each Wrong Option is Incorrect**
**Option A:** 0 mmHg is incorrect because it does not account for the pressure exerted by the brain, blood, and CSF within the cranial vault.
**Option B:** 20 mmHg is incorrect because it exceeds the normal range and may indicate increased ICP, which can lead to brain damage.
**Option C:** 5 mmHg is incorrect because it is below the normal range, but it is closer to the correct answer than option A or B.

**Clinical Pearl / High-Yield Fact**
Remember that increased ICP can be caused by a variety of factors, including head trauma, stroke, tumors, and hydrocephalus. Early recognition and management of elevated ICP are crucial to prevent brain damage and improve outcomes.

**Correct Answer: C. 10-15 mmHg**