**Core Concept**
Phenytoin, a widely used antiepileptic medication, can cause megaloblastic anemia due to its interference with folate metabolism. The patient's low hemoglobin (Hb) level and symptoms of lassitude suggest a possible hematological adverse effect.

**Why the Correct Answer is Right**
The patient's megaloblastic anemia is likely a result of phenytoin-induced folate deficiency. Estimating the Mean Corpuscular Volume (MCV) will help confirm this diagnosis, as megaloblastic anemia is characterized by an elevated MCV. This is a crucial step in the management of the patient, as it will guide further investigations and treatment. Folate supplementation may be necessary to correct the anemia.

**Why Each Wrong Option is Incorrect**
**Option A:** Chest X-ray is not directly relevant to the patient's symptoms and laboratory findings. It may be useful in evaluating other conditions, but it is not the next step in managing this patient.
**Option C:** Gamma-glutamyl transferase (GGT) is an enzyme involved in liver function tests, but it is not directly related to the patient's anemia or symptoms. Estimating GGT is not the next logical step in managing this patient.

**Clinical Pearl / High-Yield Fact**
Phenytoin-induced megaloblastic anemia is a classic example of a drug-induced hematological adverse effect. It highlights the importance of monitoring patients on long-term antiepileptic therapy for signs of anemia and folate deficiency.

**Correct Answer Line**
✓ Correct Answer: B. MCV should be estimated