**Core Concept**
Perihilar fluffy opacities on chest X-ray are indicative of pulmonary edema, which occurs due to the accumulation of fluid in the lungs. This condition is often associated with heart failure, high altitude, or other factors that increase pulmonary venous pressure.

**Why the Correct Answer is Right**
The presence of perihilar fluffy opacities is a classic radiographic finding in pulmonary edema. This occurs when there is increased hydrostatic pressure in the pulmonary capillaries, causing fluid to leak into the alveoli. The fluffy appearance is due to the accumulation of edematous fluid in the lung interstitium. The perihilar distribution is characteristic because it reflects the area of highest pressure in the pulmonary vasculature.

**Why Each Wrong Option is Incorrect**
**Option A:** This option might be tempting, but it is incorrect because interstitial lung disease typically presents with reticular or nodular patterns on chest X-ray, not fluffy opacities.
**Option B:** While pneumonia can cause lung opacities, it usually presents with more focal and patchy changes rather than the diffuse perihilar fluffy pattern seen in pulmonary edema.
**Option C:** Pleural effusion would present with a more peripheral and linear pattern on chest X-ray, rather than the fluffy opacities seen in pulmonary edema.

**Clinical Pearl / High-Yield Fact**
A key clinical correlation to remember is that perihilar fluffy opacities on chest X-ray can be a sign of acute heart failure, so a thorough cardiac evaluation is essential in such cases.

**Correct Answer: B. Pulmonary edema.**