**Core Concept**
Portal hypertension is a critical condition characterized by increased pressure in the portal vein, which can lead to severe complications such as bleeding esophageal varices. The hepatic venous pressure gradient (HVPG) is a key measurement used to assess portal pressure.

**Why the Correct Answer is Right**
The correct answer, **12 mm Hg**, is based on the widely accepted threshold for portal hypertension. Elevated portal pressure is generally defined as an HVPG greater than 5 mm Hg above the inferior vena cava pressure, but a commonly used cutoff is an HVPG of 12 mm Hg or higher. This increased pressure is often due to obstruction of portal blood flow, which can be caused by liver cirrhosis, as seen in this patient. The liver's ability to detoxify and metabolize substances is compromised when portal pressure exceeds normal levels.

**Why Each Wrong Option is Incorrect**
**Option A:** 0.15 mm Hg is an implausibly low value for portal pressure, as it is much lower than the normal range.
**Option B:** 1.5 mm Hg is also an unusually low value and does not accurately represent the threshold for portal hypertension.
**Option D:** 40 mm Hg is an excessively high value, as it would likely result in severe complications and is not a realistic threshold for portal hypertension.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the HVPG is a more accurate measurement of portal pressure than direct measurement of portal vein pressure. The HVPG takes into account the resistance in the hepatic veins and is a critical parameter in assessing the severity of portal hypertension.

**✓ Correct Answer: C. 12 mm Hg**