**Core Concept**
Pericarditis and pericardial effusion in children can be caused by various underlying conditions, including infections, autoimmune disorders, and inflammatory diseases. The least common cause among the options provided is a rare condition that affects the immune system and can cause inflammation in multiple organs.

**Why the Correct Answer is Right**
Sarcoidosis is a rare, multisystemic granulomatous disease that can cause pericarditis and pericardial effusion in children. The exact mechanism of sarcoidosis is not fully understood, but it is believed to be caused by an abnormal immune response to an unknown antigen. In the context of pericarditis, sarcoidosis can cause non-specific inflammation of the pericardium, leading to an effusion. This is a rare occurrence, especially in children, making it the least common cause among the options provided.

**Why Each Wrong Option is Incorrect**
**Option A:** Rheumatic fever is a common cause of pericarditis and pericardial effusion in children, particularly after a group A beta-hemolytic streptococcal infection. It is characterized by an autoimmune response to the streptococcal infection, leading to inflammation of the heart valves and pericardium.

**Option C:** Rheumatoid arthritis is a systemic autoimmune disease that can cause pericarditis and pericardial effusion in children, although it is relatively rare. The exact mechanism of rheumatoid arthritis is not fully understood, but it is believed to be caused by an abnormal immune response to an unknown antigen.

**Option D:** Systemic lupus erythematosus (SLE) is a multisystemic autoimmune disease that can cause pericarditis and pericardial effusion in children. SLE is characterized by an abnormal immune response to an unknown antigen, leading to inflammation in multiple organs, including the pericardium.

**Clinical Pearl / High-Yield Fact**
It is essential to consider rare conditions such as sarcoidosis when evaluating children with pericarditis or pericardial effusion, as they can mimic more common causes and require a different approach to diagnosis and management.

**✓ Correct Answer: B. Sarcoidosis**