**Question:** Megaloblastic anaemia is caused by:

A. Deficiency in folic acid
B. Deficiency in vitamin B12
C. Excessive alcohol consumption
D. Iron deficiency

**Core Concept:**
Megaloblastic anaemia is a type of anaemia characterized by the enlargement of immature red blood cells called megaloblasts, which leads to reduced red blood cell production and impaired oxygen transport. This condition is often caused by nutritional deficiencies or toxic substances affecting the synthesis of DNA and RNA in red blood cells.

**Why the Correct Answer is Right:**
Megaloblastic anaemia is primarily caused by deficiencies in folic acid (vitamin B9) and vitamin B12 (cobalamin). These vitamins are essential for the synthesis of nucleotides, specifically deoxyribonucleotides, which are required for DNA and RNA synthesis in the nucleated red blood cells. Deficiencies in these vitamins hinder the proper development of red blood cells, leading to megaloblastic anaemia.

**Why Each Wrong Option is Incorrect:**
Option A (deficiency in folic acid) is partially correct, as folic acid plays a crucial role in DNA synthesis. However, the main cause of megaloblastic anaemia lies in the deficiency of vitamin B12, which is involved in the salvage pathway of DNA synthesis.

Option B (deficiency in vitamin B12) is more relevant since it directly impacts the synthesis of DNA and RNA, leading to impaired development of red blood cells, whereas folic acid deficiency mainly affects DNA synthesis.

Option C (excessive alcohol consumption) is a risk factor for megaloblastic anaemia due to malabsorption of vitamin B12 in the presence of alcoholic gastritis. However, it is not the direct cause of megaloblastic anaemia itself.

Option D (iron deficiency) is incorrect because iron deficiency anaemia primarily affects the production of haemoglobin, leading to a decrease in red blood cells, whereas megaloblastic anaemia is characterized by the presence of megaloblastic cells.

**Clinical Pearl:** Megaloblastic anaemia should always be suspected in patients presenting with symptoms like weakness, fatigue, pallor, and macrocytic (large) red blood cells. Investigations like serum vitamin B12 and folic acid levels can confirm the diagnosis. Treatment involves replacing the deficient vitamin, usually vitamin B12, to restore red blood cell production and alleviate the symptoms.