**Question:** The primary involvement of which organ is so far not reported to be affected by sarcoidosis is-

A. Heart
B. Brain
C. Kidney
D. Skin

**Core Concept:**
Sarcoidosis is a multisystem granulomatous inflammatory disease of unknown cause, affecting various organs and tissues. The disease primarily involves the lungs, lymph nodes, and skin, and can also involve the eyes, joints, liver, and kidneys. Sarcoidosis typically affects young adults and is more common in African Americans.

**Why the Correct Answer is Right:**
Sarcoidosis primarily involves the respiratory system (pulmonary granulomas) and the skin (cutaneous lesions). Involvement of the heart is rare, with myocardial involvement reported in less than 5% of cases. The heart is protected by the conduction system, which is located in the right atrium and ventricles, making it less susceptible to granulomatous inflammation.

**Why Each Wrong Option is Incorrect:**
The brain (option B) is protected by the blood-brain barrier, which prevents the entry of many molecules, including cytokines and immune cells. Sarcoidosis affecting the brain is extremely rare, with only a few cases reported. The involvement of the kidney (option C) is also uncommon, affecting less than 10% of patients. Sarcoidosis can cause kidney involvement through granulomatous inflammation leading to renal parenchymal disease and glomerulonephritis. However, the kidney is not as directly affected as the lungs and skin. The skin (option D) is affected in up to 80% of patients with sarcoidosis, making it the most commonly affected organ.

**Clinical Pearl:**
While sarcoidosis primarily affects the lungs, skin, and lymph nodes, it is essential to consider the involvement of other organs like the heart and kidney, especially in patients presenting with specific symptoms or signs. A thorough clinical examination and appropriate investigations can help identify these rare but crucial manifestations of sarcoidosis.

**Correct Answer:**
The correct answer is B, the brain. The heart (A) and kidney (C) are less commonly involved, and the skin (D) is the most commonly affected organ. The involvement of the brain is extremely rare, making option B the correct answer.