Lymphocytosis with atypical lymphocytes are seen in infection with –
**Core Concept**
Lymphocytosis with atypical lymphocytes is a characteristic hematological finding in certain viral infections, reflecting the body's immune response to the pathogen.
**Why the Correct Answer is Right**
The presence of atypical lymphocytes in lymphocytosis is a hallmark of viral infections, particularly those caused by cytomegalovirus (CMV), Epstein-Barr virus (EBV), and human immunodeficiency virus (HIV). These viruses induce lymphocyte activation, proliferation, and differentiation, resulting in the production of atypical lymphocytes. The exact mechanism involves the binding of viral antigens to MHC class I molecules on the surface of infected lymphocytes, leading to the activation of T cells and the subsequent production of atypical lymphocytes.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because lymphocytosis with atypical lymphocytes is not typically seen in bacterial infections, such as those caused by Streptococcus pyogenes or Staphylococcus aureus.
* **Option B:** This option is incorrect because while lymphocytosis can be seen in some cases of fungal infections, such as histoplasmosis, it is not typically associated with atypical lymphocytes.
* **Option C:** This option is incorrect because lymphocytosis with atypical lymphocytes is not typically seen in parasitic infections, such as malaria or toxoplasmosis.
**Clinical Pearl / High-Yield Fact**
In patients with lymphocytosis and atypical lymphocytes, it is essential to consider viral infections, particularly CMV, EBV, and HIV, as potential causes. A thorough history, physical examination, and laboratory tests, including serology and PCR, can help establish the diagnosis.
**Correct Answer:** C.