**Core Concept**
Coxsackie B virus is a member of the enterovirus family, which is a common cause of viral myocarditis and pericarditis, particularly in infants and young children. This condition is characterized by inflammation of the heart muscle (myocarditis) and the membrane surrounding the heart (pericarditis).

**Why the Correct Answer is Right**
Coxsackie B virus enters the body through the gastrointestinal tract and replicates in the lymphoid tissues, leading to viral myocarditis and pericarditis. This occurs due to the direct cytopathic effect of the virus on cardiac myocytes and the subsequent activation of immune cells, resulting in an inflammatory response. The heart's inability to maintain a stable cardiac output and rhythm leads to various clinical manifestations.

**Why Each Wrong Option is Incorrect**
**Option A:** Coxsackie A virus is primarily associated with hand, foot, and mouth disease, a common viral illness in children, but it is not typically linked to myocarditis and pericarditis.
**Option C:** Mumps virus can cause myocarditis and pericarditis, but it is less common compared to Coxsackie B virus in infants and young children.
**Option D:** Pox virus is not typically associated with myocarditis and pericarditis in infants and young children.

**Clinical Pearl / High-Yield Fact**
Viral myocarditis and pericarditis in infants can be challenging to diagnose, and a high index of suspicion is required. Clinical manifestations may include tachycardia, tachypnea, and cardiac dysfunction, and the diagnosis is often made based on a combination of clinical findings, electrocardiogram, and echocardiogram.

**✓ Correct Answer: B. Coxsackie B**