## **Core Concept**
The patient's presentation of generalized lymphadenopathy, elevated WBC count of 20,000/mm^3, and a blood film showing >70% mature-looking lymphocytes in a patient over 70 years old is highly suggestive of a lymphoproliferative disorder, specifically **Chronic Lymphocytic Leukemia (CLL)**. CLL is a type of cancer that affects the blood and bone marrow, characterized by the production of an excessive number of immature white blood cells, known as lymphocytes.

## **Why the Correct Answer is Right**
The clinical presentation and laboratory findings point towards CLL, which is the most common type of leukemia in adults, especially in those over 60 years of age. The diagnosis of CLL is often confirmed by **flow cytometry** which helps in identifying the specific type of lymphocytes (B cells in CLL) and assessing the expression of specific markers such as CD5 and CD23. This is crucial for differentiating CLL from other lymphoproliferative disorders like mantle cell lymphoma.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While a **bone marrow biopsy** can provide supportive evidence for CLL by showing infiltration of lymphocytes, it is not the immediate next step after a suggestive blood film.
- **Option B:** **Lymph node biopsy** might be considered if there's a need to assess lymphadenopathy but given the typical presentation of CLL, a more specific diagnostic test like flow cytometry would be preferred over a biopsy for initial diagnosis.
- **Option C:** **CT scan** might be useful for staging but is not the next best investigation to confirm the diagnosis after a suggestive blood film.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that CLL is diagnosed based on the presence of **>5,000/μL clonal B lymphocytes** in the peripheral blood and the specific immunophenotypic markers (e.g., CD5+, CD23+, and low expression of CD20 and CD22). Early diagnosis often occurs incidentally during routine blood tests.

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