**Question:** All of the following are causes of "lupus" except

A. Hypothyroidism
B. Systemic sclerosis
C. Viral infections
D. Calcium deficiency

**Core Concept:**
Lupus is a group of autoimmune diseases characterized by inflammation and tissue damage caused by the body's immune system attacking its own cells and tissues. It is classified into two types:
1. Systemic Lupus Erythematosus (SLE) - a severe form affecting multiple organs
2. Mixed Connective Tissue Disease (MCTD) - a combination of symptoms from SLE, rheumatoid arthritis, and dermatomyositis

**Why the Correct Answer is Right:**
D. Calcium deficiency is not a cause of lupus. Lupus is an autoimmune disease, which means it is caused by an abnormal immune response rather than a deficiency or excess of specific nutrients. In contrast, calcium deficiency can lead to osteoporosis, rickets, or other nutrient deficiencies but not lupus.

**Why Each Wrong Option is Incorrect:**
A. Hypothyroidism (underactive thyroid gland) is unrelated to lupus as it affects the thyroid hormone levels, not the immune system's functioning. Hypothyroidism can cause symptoms like fatigue, weight gain, and constipation, but not lupus.

B. Systemic sclerosis (scleroderma) is a connective tissue disease characterized by excessive collagen production, leading to skin hardening and organ involvement. It is distinct from lupus and not a cause of lupus.

C. Viral infections, such as Epstein-Barr virus, parvovirus, or human herpesvirus, can trigger lupus flares or exacerbate existing lupus symptoms but are not the primary cause of lupus.

**Clinical Pearl:**
It is essential for medical students and practitioners to understand the concept of autoimmune diseases like lupus, which are caused by the body's immune system attacking its own tissues, differentiating them from conditions related to nutrient deficiencies or infections. This knowledge helps in proper diagnosis, appropriate management, and explaining the cause to patients.