Ans. is 'a' i.e., Infectious mononucleiosis Here the question seems incomplete. Most probably they have asked about atypical lymphocytosis. Infectious mononucleiosis is characterized by classic triad of : Fever, Sore throat, Posterior cervical lymphadenopathy. This classic triad is seen in > 80% of the patients. The W.B.C. count is characteristically increased. Lymphocytosis is demonstrable with 10-20% of them being atypical. The atypical lymphocytes are called Downey cell. These cells have higher cytoplasm to nucleus ratio.