**Core Concept**
The patient's presentation of generalized lymphadenopathy and a blood film showing 70% immature looking lymphocytes suggests a diagnosis of acute lymphoblastic leukemia (ALL). This condition is characterized by the clonal expansion of immature lymphoid cells, which can lead to bone marrow failure and increased risk of infections.

**Why the Correct Answer is Right**
The next best investigation for ALL is a bone marrow biopsy. This procedure allows for the collection of a sample of bone marrow cells, which can be examined for the presence of leukemic cells. A bone marrow biopsy can provide valuable information on the type and maturity of the lymphoid cells, as well as the extent of bone marrow involvement. This information is crucial for diagnosing and staging ALL. **Flow cytometry** and **cytogenetic analysis** may also be performed on the bone marrow sample to further characterize the leukemic cells and guide treatment.

**Why Each Wrong Option is Incorrect**
**Option A:** Peripheral blood smear is already mentioned in the question, so it's not the next best investigation. It's more of a diagnostic tool used to identify the presence of immature lymphocytes in the blood.

**Option B:** CT scan of the neck, chest, and abdomen may be helpful in evaluating the extent of disease spread, but it's not the next best investigation for diagnosing ALL.

**Option C:** Liver function tests (LFTs) may be elevated in ALL due to liver involvement, but they are not diagnostic and do not provide information on the bone marrow involvement.

**Clinical Pearl / High-Yield Fact**
In patients presenting with acute leukemia, it's essential to perform a bone marrow biopsy as soon as possible to confirm the diagnosis and guide treatment. Delaying this investigation can lead to delays in initiating effective therapy.

**Correct Answer:** C. Bone marrow biopsy.