**Core Concept**
Infectious mononucleosis (IMN) is a viral illness caused by Epstein-Barr virus (EBV) characterized by fever, lymphadenopathy, and hepatosplenomegaly. However, the absence of hepatomegaly in this case narrows down the differential diagnosis. The presence of lympho plasmacytoid cells in the peripheral blood smear is a key diagnostic feature of IMN.

**Why the Correct Answer is Right**
Infectious mononucleosis is caused by EBV, which infects B lymphocytes. The virus triggers an immune response, leading to the proliferation of lymphocytes, including lympho plasmacytoid cells. These cells are a hallmark of IMN and can be seen in up to 50% of patients. The presence of >20% lympho plasmacytoid cells in the peripheral blood smear is a strong indicator of IMN.

**Why Each Wrong Option is Incorrect**
**Option A:** Tuberculosis (TB) is a bacterial infection that can cause lymphadenopathy, but it is typically associated with a more chronic course and is less likely to cause the presence of lympho plasmacytoid cells in the peripheral blood smear.

**Option C:** Acute lymphoblastic leukemia (ALL) is a malignancy of the lymphoid lineage, but it would typically present with a more severe anemia, thrombocytopenia, and a higher white blood cell count, which is not seen in this case.

**Option D:** Influenza is a viral respiratory illness that can cause fever and lymphadenopathy, but it is not associated with the presence of lympho plasmacytoid cells in the peripheral blood smear.

**Clinical Pearl / High-Yield Fact**
Infectious mononucleosis can be diagnosed based on the presence of lympho plasmacytoid cells in the peripheral blood smear, along with clinical symptoms such as fever and lymphadenopathy. However, it's essential to note that EBV serology can be used to confirm the diagnosis, especially in cases where the clinical presentation is atypical.

**✓ Correct Answer: B. Infectious Mononucleosis**