**Core Concept**
The question tests the ability to diagnose hematological malignancies based on immunophenotyping. **Chronic Lymphocytic Leukemia (CLL)** and **Non-Hodgkin Lymphoma (NHL)** are key considerations when lymphadenopathy and splenomegaly are present. Immunophenotyping helps differentiate between various types of lymphoid neoplasms.

**Why the Correct Answer is Right**
Given the positivity for **CD19**, **CD79b**, and **FMC7**, the diagnosis points towards a B-cell lymphoproliferative disorder. **CD19** and **CD79b** are B-cell specific antigens, while **FMC7** is often positive in mantle cell lymphoma and some cases of CLL, but typically negative in CLL. This pattern suggests a **B-cell Non-Hodgkin Lymphoma**, with mantle cell lymphoma being a strong candidate due to the specific markers mentioned.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the specific immunophenotypic markers mentioned do not align with the typical profile for this option.
**Option B:** Incorrect as it usually lacks FMC7 positivity.
**Option D:** Incorrect due to the lack of specific markers that would point towards this diagnosis.

**Clinical Pearl / High-Yield Fact**
Remember, **CD5** and **CD23** positivity, along with **CD19** and **CD79b**, can help differentiate CLL from other B-cell lymphomas, as CLL is typically **CD5** and **CD23** positive, while mantle cell lymphoma is **CD5** positive but **CD23** negative.

**Correct Answer:** D. Chronic Lymphocytic Leukemia