Question

Cerebral blood flow is regulated by all except:

• A. Blood pressure
• B. Arterial PCO₂
• C. Potassium ions
• D. A & C

Answer 1

The Correct Option is D

Approach this by ranking the regulators of cerebral blood flow. The single most powerful controller is arterial PCO2: a rise in CO2 dilates cerebral vessels strongly and raises flow, so option (b) is a genuine regulator and cannot be the exception. That immediately narrows the answer to the choices involving blood pressure and potassium.
Cerebral blood flow is famously buffered by autoregulation, staying nearly constant across a mean arterial pressure of roughly 60-150 mmHg, so within the physiological range blood pressure is not an effective moment-to-moment regulator. Potassium is only a weak metabolic contributor compared with PCO2 and adenosine. Since the stem asks for what does not regulate flow, the combined option naming both blood pressure and potassium ions, option (d), is the intended answer. PCO2 remains the dominant true regulator.
Ref: Principles of Medical Physiology; RK Marya.