## **Core Concept**
The patient's presentation of generalized lymphadenopathy, elevated WBC count with a predominance of mature-looking lymphocytes (>70%) in the blood film, suggests a lymphoproliferative disorder. Given the age of the patient (>70 years), chronic lymphocytic leukemia (CLL) is a likely diagnosis. CLL is characterized by the clonal proliferation of mature-appearing lymphocytes.

## **Why the Correct Answer is Right**
The correct approach involves confirming the diagnosis and assessing the extent of disease. For CLL, specific investigations are crucial for diagnosis and staging. Flow cytometry on peripheral blood is essential as it helps in identifying the specific surface markers on lymphocytes (e.g., CD5, CD20, CD23), which are critical for diagnosing CLL. This test aids in differentiating CLL from other lymphoproliferative disorders.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While a bone marrow biopsy can provide valuable information on cellularity and infiltration by lymphocytes, it is not the immediate next step after a blood film suggestive of CLL, especially with the availability of less invasive diagnostic techniques.
- **Option B:** Imaging studies like CT scans can be useful for assessing lymphadenopathy and organomegaly but are not the first line in confirming the diagnosis of a specific lymphoproliferative disorder like CLL.
- **Option C:** Although a lymph node biopsy can be diagnostic, it is more invasive compared to flow cytometry on peripheral blood and is usually reserved for cases where the diagnosis is unclear or when specific features suggest a need for tissue architecture assessment.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that CLL is diagnosed based on the presence of a specific immunophenotypic profile (typically CD5+, CD20+, CD23+, and low expression of CD79b and CD81) in addition to clinical findings and a high lymphocyte count. Flow cytometry is pivotal in establishing this diagnosis.

## **Correct Answer:** . **Flow cytometry**