**Question:** A characteristic histologic feature of class IV lupus nephritis is

A. Mesangial hypercellularity
B. Endocapillary hypercellularity
C. Fibrinoid necrosis
D. Arterial involvement

**Core Concept:** Lupus nephritis is a complication of systemic lupus erythematosus (SLE) that affects the kidneys. The disease is classified into six classes based on the extent and severity of renal lesions. Histology plays a vital role in classifying lupus nephritis, which helps guide treatment decisions.

**Why the Correct Answer is Right:** Class IV lupus nephritis is characterized by endocapillary proliferation, also known as endocapillary hypercellularity. This feature represents the increased number of cells (predominantly lymphocytes) within the glomerular capillaries, resulting in a swollen mesangium and glomerular basement membrane thickening.

**Why Each Wrong Option is Incorrect:**

A. Mesangial hypercellularity is a feature of class II and III lupus nephritis, indicating an increased number of cells in the mesangium.
B. Endocapillary hypercellularity is the correct answer, not endocapillary hypercellularity.
C. Fibrinoid necrosis is a feature of class V lupus nephritis, indicating degeneration and necrosis of the glomerular capillary walls.
D. Arterial involvement is a feature of class V lupus nephritis, representing inflammation and damage to the arterial wall.

**Clinical Pearl:** Understanding the histopathological classification of lupus nephritis is crucial for accurate diagnosis and appropriate management. The different classes (I-VI) are based on the degree and distribution of glomerular and tubulointerstitial lesions.

**Correct Answer:** B. Endocapillary hypercellularity