**Question:** According to WHO, the feature of class II lupus is:

A. Presence of anti-double-stranded DNA antibodies
B. Presence of anti-histone antibodies
C. Presence of anti-Smith antibodies
D. Presence of anti-RNP antibodies

**Core Concept:**
Lupus is a chronic autoimmune disease that can affect various organs and systems in the body. Classification of lupus is based on the presence of specific autoantibodies produced by the immune system against the patient's own cellular components. The American College of Rheumatology and the World Health Organization (WHO) have developed different classification systems for lupus, with minor differences in criteria.

**Why the Correct Answer is Right:**
Class II lupus, as defined by the WHO, is characterized by the presence of anti-double-stranded DNA antibodies. These antibodies target the DNA molecule, which is essential for cell replication and gene expression. The production of anti-double-stranded DNA antibodies is a hallmark of class II lupus, indicating an active disease state and a poor prognosis.

**Why Each Wrong Option is Incorrect:**
A. Presence of anti-histone antibodies (option B) is associated with class III lupus, not class II.
B. Presence of anti-histone antibodies (option B) is a feature of class III lupus, not class II. Histones are proteins that help package and organize DNA in the nucleus of cells.
C. Presence of anti-Smith antibodies (option C) is specific to class IV lupus, not class II. Smith antigens are involved in RNA synthesis and processing.
D. Presence of anti-RNP antibodies (option D) is associated with class IV lupus, not class II. RNP antigens are involved in RNA and protein synthesis.

**Clinical Pearl:**
It is crucial for medical students and residents to understand the various classification systems of lupus and the specific antibodies associated with each class. This knowledge aids in accurate diagnosis, prognosis, and appropriate management of lupus patients.