Examination of a lymph node from the neck of a 26-year-old man with bilateral cervical lymphadenopathy reveals total effacement of nodal architecture, and at higher power, the characteristic cell shown below. Which of the following additional studies is best suited for confirmation of diagnosis in this case?

The picture shows prominent Reed-Sternberg cell (RS cell) which is typical of classical Hodgkin lymphoma. The diagnosis is based on the biopsy findings, and can be confirmed by doing immunohistochemical markers on the lymph node biopsy taken. RS cells are positive for CD15, CD30 and negative for CD20, CD45. These cells also show weak positivity for PAX-5. The other tests are not required for confirmation. Immunohistochemistry is best suited for confirmation of diagnosis in cases of Hodgkin lymphoma.