A college student seeks medical care for extreme fatigue and swollen lymph nodes in the neck. The WBC count is 11.3 ´ 109/L. The differential shows a relative and absolute lymphocytosis

Many of the lymphocytes are larger than normal, with a basophilic cytoplasm indented by the red blood cells (RBCs). The nuclei are less clumped than normal and some have nucleoli. The rest of the complete blood count (CBC) is normal. What is the most likely diagnosis?

a. Infectious lymphocytosis
b. Infectious mononucleosis
c. Lymphoma
d. Lymphocytic leukemia

ANS: B
The description of these cells is that of a reactive lymphocyte. They are a normal response to viral infection. Infectious mononucleosis characteristically shows a lymphocytosis with many reactive T lymphocytes. The age, symptoms, and physical findings also support infectious mononucleosis.