**Core Concept**
Grey hepatization is a stage in the evolution of pulmonary congestion and infarction, characterized by the filling of alveoli with a mixture of blood, inflammatory cells, and fibrin. This stage occurs after the initial exudate phase and precedes red hepatization and resolution.

**Why the Correct Answer is Right**
In grey hepatization, the alveoli are filled with **fibrin**, red blood cells, and inflammatory cells. The fibrin forms a web-like matrix that traps red blood cells and cellular debris, giving the lung tissue a greyish, firm, and hepatoid appearance. This phase follows the early exudative stage and is driven by increased capillary permeability and fibrin deposition in the alveolar spaces. Fibrin is the key component that gives the stage its name and clinical appearance.

**Why Each Wrong Option is Incorrect**
Option A: WBCs fill the alveoli — This occurs in the early exudative phase, not grey hepatization. WBCs are present but not the dominant component.
Option B: RBCs fill the alveoli — RBCs are present in abundance, but they are not the defining feature. The stage is defined by fibrin, not just RBCs.
Option C: Organisms fill the alveoli — This refers to bacterial infection, which is not characteristic of grey hepatization and is unrelated to pulmonary congestion.

**Clinical Pearl / High-Yield Fact**
Grey hepatization is the second stage in alveolar filling during pulmonary infarction or congestion. Remember: **"Fibrin is the hallmark"** — without fibrin, the stage would not be classified as grey hepatization.

✓ Correct Answer: D. Accumulation of fibrin