Which one of the following infections is characterized by an increased TLC with absolute lymphocytosis?
**Core Concept**
Toxic granulocytosis and absolute lymphocytosis are often seen in viral infections, particularly those caused by viruses that affect the bone marrow or lymphoid tissues. In these cases, the total leukocyte count (TLC) increases due to an increase in the number of lymphocytes.
**Why the Correct Answer is Right**
In viral infections such as infectious mononucleosis (caused by Epstein-Barr virus), the bone marrow is affected, leading to an increase in the production of lymphocytes. This results in an absolute lymphocytosis, where the lymphocyte count is significantly higher than the normal range. The TLC also increases due to the overall increase in white blood cell production. The mechanism behind this is the stimulation of the bone marrow by the viral infection, leading to an increase in the proliferation and differentiation of hematopoietic stem cells into lymphocytes.
**Why Each Wrong Option is Incorrect**
**Option A:** Tuberculosis (TB) typically presents with a decreased TLC due to the suppression of the bone marrow by the TB organisms. While TB can cause an increase in lymphocytes, it is not characterized by absolute lymphocytosis.
**Option B:** Fungal infections such as histoplasmosis can cause an increase in the TLC, but this is typically due to an increase in granulocytes (such as neutrophils) rather than lymphocytes.
**Option C:** Bacterial infections such as pneumonia can cause an increase in the TLC, but this is typically due to an increase in granulocytes (such as neutrophils) rather than lymphocytes.
**Clinical Pearl / High-Yield Fact**
Infectious mononucleosis is often associated with fatigue, fever, sore throat, and lymphadenopathy. It is essential to consider this diagnosis in patients presenting with these symptoms, especially during the spring and fall seasons when the virus is more prevalent.
**Correct Answer: C. Infectious mononucleosis**