**Core Concept**
The volatility of an anesthetic agent is directly proportional to its blood/gas partition coefficient, which affects the rate of induction and recovery. The blood/gas partition coefficient is influenced by various factors, including the agent's molecular weight, lipid solubility, and the presence of blood flow. In this context, the statement suggests a relationship between the volatility of an anesthetic agent and the flow in the portal vein.

**Why the Correct Answer is Right**
The correct answer relates to the concept of hepatic blood flow and its impact on the pharmacokinetics of anesthetic agents. The portal vein carries deoxygenated blood from the gastrointestinal tract, spleen, and pancreas to the liver. The liver plays a crucial role in metabolizing many anesthetic agents, including those with high lipid solubility. Therefore, a reduction in portal vein flow would decrease the liver's ability to metabolize these agents, leading to increased blood levels and potentially prolonged recovery.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the question specifically asks about the portal vein, which is not directly related to the hepatic artery's blood flow.

**Option B:** This option is incorrect because the question asks about the portal vein, not the hepatic artery, which supplies oxygenated blood to the liver.

**Option C:** This option is incorrect because it is a distractor and does not relate to the concept of anesthetic volatility and portal vein flow.

**Option D:** This option is incorrect because it is a distractor and does not relate to the concept of anesthetic volatility and portal vein flow.

**Clinical Pearl / High-Yield Fact**
The liver's ability to metabolize anesthetic agents is influenced by blood flow, with a 10% decrease in blood flow resulting in a 50% decrease in metabolic activity. This is a critical consideration when managing patients with liver disease or during procedures that may compromise hepatic blood flow.

**Correct Answer: C. 90% (of portal flow is reduced at a mean arterial pressure of 90 mmHg)**