**Core Concept**
Kerley B lines are thin, horizontal lines seen on chest X-ray that represent fluid accumulation in the interstitial spaces of the lungs, typically due to pulmonary edema or left heart failure. They are best visualized at the pleural surface where the interstitial space is compressed by the pleura.

**Why the Correct Answer is Right**
Kerley B lines form due to fluid accumulation in the subpleural interstitial spaces, particularly at the **pleural surface**. These lines appear as fine, linear opacities at the **cardiophrenic angle** and are most prominent in the **peripheral lung zones**. The fluid accumulates in the interstitial spaces between the alveoli and the pleura, making the pleural surface the anatomical site where these lines are formed and visible.

**Why Each Wrong Option is Incorrect**
Option A: The apex of the lung is where Kerley A lines are seen, not Kerley B lines. Kerley B lines are not located at the apex.
Option B: While Kerley B lines are often seen near the cardiophrenic angle, this is a **location**, not the **anatomical site** of formation. The actual site of fluid accumulation is the pleural surface, not the angle itself.
Option C: Lung fissures are associated with Kerley A lines or fissure-related opacities, not Kerley B lines.

**Clinical Pearl / High-Yield Fact**
Kerley B lines are a classic sign of **left-sided heart failure** and pulmonary edema. Their presence indicates **interstitial fluid accumulation** at the pleural surface and should prompt evaluation for cardiac pathology.

✓ Correct Answer: D. Pleural surface