**Core Concept**
Myxoma is the most common type of primary cardiac tumor in adults, accounting for approximately 50% of all primary cardiac tumors. It is a benign neoplasm of the heart, often presenting as a solitary, well-circumscribed mass within the atria.

**Why the Correct Answer is Right**
Myxomas are typically composed of mesenchymal cells, which produce mucoid and myxoid matrix, leading to the characteristic appearance of these tumors. They are often associated with a variety of clinical symptoms, including dyspnea, palpitations, and fatigue, due to obstruction of blood flow or embolization of tumor fragments. The exact mechanism of myxoma formation is not well understood, but it is thought to arise from the endocardium or epicardium.

**Why Each Wrong Option is Incorrect**
**Option A:** Rhabdomyosarcoma is a malignant tumor of striated muscle that can arise in various locations, including the head and neck, genitourinary tract, and extremities. It is not typically associated with the heart.

**Option C:** Fibroma is a rare, benign tumor of fibrous tissue that can occur in various locations, including the heart. However, it is much less common than myxoma and typically presents as a solitary, well-circumscribed mass within the ventricles.

**Option D:** Lymphoma is a type of malignant tumor that arises from lymphoid cells and can occur in various locations, including the heart. However, it is not typically associated with the heart as a primary malignancy.

**Clinical Pearl / High-Yield Fact**
Myxoma is the most common primary cardiac tumor, and its diagnosis often requires histological examination of the tumor tissue. On imaging studies, myxoma typically appears as a well-circumscribed, homogeneous mass within the atria.

**✓ Correct Answer: B. Myxoma**