## **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 person over 70 years old is highly suggestive of a lymphoproliferative disorder, specifically **Chronic Lymphocytic Leukemia (CLL)**.

## **Why the Correct Answer is Right**
CLL is characterized by the clonal proliferation of mature-appearing lymphocytes in the blood, bone marrow, and lymphoid tissues. The diagnosis of CLL often involves demonstrating the presence of specific cell surface markers (e.g., CD5 and CD23) on the lymphocytes. The **next investigation that should be done** to confirm the diagnosis and assess the prognosis would logically involve **flow cytometry** to identify these markers. This test helps differentiate CLL from other lymphoproliferative disorders.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While a **bone marrow biopsy** can provide valuable information on the extent of marrow involvement, it is not the immediate next step after identifying a high percentage of mature lymphocytes in the blood film.
- **Option B:** A **lymph node biopsy** could provide diagnostic information but is more invasive and not typically the next step after a suggestive blood film and clinical presentation.
- **Option C:** Although **imaging studies** (like CT scans) can help assess the extent of lymphadenopathy and organomegaly, they do not provide a specific diagnosis at the cellular level.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that CLL cells typically co-express **CD5 and CD23**, which helps differentiate it from mantle cell lymphoma (which expresses CD5 but not CD23). Flow cytometry for these markers is crucial for diagnosis.

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