This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Physiology
Cerebral blood flow is regulated by all, EXCEPT: 
Select and explain. 
A. Intracranial pressure, B. Aerial PC02, C. Potassium ions, D. Cerebral metabolic rate
C: Potassium ions
Answer: C: Potassium ions 
Explanation: Factors affecting overall cerebral blood flow include: Intracranial pressure Local constriction & dilation of cerebral aerioles Mean aerial pressure at brain level Viscosity of blood Mean venous pressure at brain level The most impoant extrinsic influences on CBF are respiratory gas tensions paicularly aerial PC02. Cerebral blood flow is directly propoional to aerial PC02 between tensions of 20 and 80 mm of Hg Ref: Guyton 12th Ed.