**Core Concept**
The question tests the student's understanding of the physiological relationship between left atrial pressure and the onset of pulmonary edema following acute left ventricular failure. Left atrial pressure is a critical determinant of fluid accumulation in the lungs.

**Why the Correct Answer is Right**
When left ventricular function fails acutely, the left ventricle is unable to effectively pump blood out of the left atrium. As a result, the left atrial pressure increases. Pulmonary edema typically begins to appear when left atrial pressure approaches 20 mm Hg. This is because at this pressure, fluid begins to leak out of the pulmonary capillaries and into the alveoli, leading to the characteristic symptoms of pulmonary edema. The increase in left atrial pressure is directly related to the failure of the left ventricle to pump effectively, resulting in a backlog of blood in the left atrium.

**Why Each Wrong Option is Incorrect**
**Option A:** 7 mm Hg is a relatively low pressure and would not typically lead to pulmonary edema. At this pressure, the left atrium is still able to effectively pump blood into the left ventricle.
**Option B:** 15 mm Hg is still a relatively low pressure and would likely not cause significant fluid accumulation in the lungs. The left atrium would still be able to pump blood into the left ventricle at this pressure.
**Option D:** 30 mm Hg is a higher pressure than necessary for pulmonary edema to occur. At this pressure, the left atrium would be severely backlogged, and fluid would likely be leaking out of the pulmonary capillaries at a significant rate.

**Clinical Pearl / High-Yield Fact**
A key clinical correlation is that the onset of pulmonary edema is often preceded by a significant increase in jugular venous pressure (JVP) and peripheral edema. This is because the failure of the left ventricle to pump effectively leads to a backlog of blood in the left atrium, which in turn increases the pressure in the pulmonary veins and capillaries, causing fluid to leak out into the lungs.

**✓ Correct Answer: C. 20 mm Hg**