**Core Concept**
Hydrostatic pulmonary edema occurs due to increased hydrostatic pressure in the pulmonary capillaries, leading to fluid transudation into the alveoli. This condition is often seen in patients with left heart failure or fluid overload states.

**Why the Correct Answer is Right**
The correct answer is related to the concept of Pulmonary Capillary Wedge Pressure (PCWP). PCWP is a measure of the pressure in the left atrium, which is reflective of the left ventricular end-diastolic pressure. When PCWP is elevated, it indicates increased hydrostatic pressure in the pulmonary capillaries, leading to fluid transudation into the alveoli. This is because the capillary hydrostatic pressure exceeds the oncotic pressure, allowing fluid to leak out of the capillaries and into the alveolar space.

**Why Each Wrong Option is Incorrect**
**Option A:** A PCWP of 10 mmHg is typically within normal limits and would not cause fluid to leak into the alveoli.
**Option B:** A PCWP of 20 mmHg may cause some fluid to leak into the alveoli, but it is not typically associated with hydrostatic pulmonary edema.
**Option C:** A PCWP of 30 mmHg is significantly elevated and would likely cause significant fluid to leak into the alveoli, leading to hydrostatic pulmonary edema.

**Clinical Pearl / High-Yield Fact**
A PCWP of 18-20 mmHg is often used as a threshold for diagnosing hydrostatic pulmonary edema, but it's essential to consider other factors such as the patient's clinical presentation and radiographic findings.

**Correct Answer: C. 30 mmHg is the typical value where fluid in alveoli is seen.**