**Core Concept**
Tuberculosis (TB) in children often presents with nonspecific symptoms such as fever, lymphadenopathy, and sore throat, making diagnosis challenging. The presence of lympho-plasmacytoid cells in the peripheral blood smear is a key diagnostic clue for TB.

**Why the Correct Answer is Right**
The lympho-plasmacytoid cells in the peripheral blood smear are indicative of a chronic infection, which is consistent with TB. In TB, the spleen and lymph nodes are activated, leading to the production of lymphocytes and plasma cells. These cells can be seen in the peripheral blood, especially in children with TB lymphadenitis. The presence of these cells, along with the clinical presentation of fever, lymphadenopathy, and a history of sore throat, supports the diagnosis of TB.

**Why Each Wrong Option is Incorrect**
**Option A:** Influenza typically presents with acute onset of fever, cough, and respiratory symptoms, which does not match the chronic presentation of this case.
**Option C:** Infectious mononucleosis, caused by Epstein-Barr virus, typically presents with fever, sore throat, and lymphadenopathy, but the peripheral blood smear would show atypical lymphocytes, not lympho-plasmacytoid cells.
**Option D:** Acute lymphoblastic leukemia (ALL) would show a significant increase in blasts in the peripheral blood smear, which is not mentioned in this case.

**Clinical Pearl / High-Yield Fact**
In children, TB lymphadenitis often presents with painless, non-tender lymphadenopathy, and the presence of lympho-plasmacytoid cells in the peripheral blood smear is a key diagnostic clue. This condition should be considered in the differential diagnosis of chronic lymphadenopathy in children, especially with a history of exposure to TB.

**✓ Correct Answer: B. Tuberculosis**