This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
A 5-years old child was presented with proptosis. Microscopic examination of the mass revealed a round cell tumor positive for Desmin immunohistochemical marker. Most likely diagnosis is ? 
Select and explain. 
A. Leukemia, B. Embryonal rhabdomyosarcoma, C. Lymphoma, D. Primitive Neuroectodermal Tumor (PNET)
B: Embryonal rhabdomyosarcoma
Answer: B: Embryonal rhabdomyosarcoma 
Explanation: Ans. B. Embryonal rhabdomyosarcomaRef: Robbins & Cotran, lh ed., pg. 1220-1222ln the four options provided , all four show morphology of a round cell tumour.However Desmin positivity is seen only in cases of rhabdomyosarcoma.In Embryonal rhabdomyosarcoma, the tumor cells mimic skeletal muscle at various stages of embryogenesis and consist of sheets of both prirnitive round and spindled cells in a myxoid stroma.Immunohistochemistry for desmin and Myogenin confirm the rhabdomyoblastic differentiation.