**Core Concept**
Macrocytic anaemia is characterized by large red blood cells with a decreased number of red blood cells in the blood, leading to inadequate oxygen delivery to tissues. This condition can be caused by various factors, including deficiencies in vitamin B12 or folate, which are essential for DNA synthesis and red blood cell production.

**Why the Correct Answer is Right**
Vitamin B12 and folate deficiencies are the most common causes of macrocytic anaemia. Vitamin B12 is necessary for the synthesis of DNA, and a deficiency can lead to megaloblastic anaemia, where red blood cells are abnormally large. Folate is also crucial for DNA synthesis, and a deficiency can cause a similar megaloblastic anaemia. Other causes of macrocytic anaemia include alcoholism, which can lead to folate deficiency, and certain medications such as methotrexate, which can interfere with folate metabolism.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is a common cause of macrocytic anaemia.
**Option B:** This option is incorrect because it is a less common cause of macrocytic anaemia, but still a recognized cause.
**Option C:** This option is incorrect because it is not a typical cause of macrocytic anaemia; it is more commonly associated with microcytic anaemia.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that macrocytic anaemia can be caused by both nutritional deficiencies (e.g., vitamin B12 or folate deficiency) and certain medications (e.g., methotrexate). A thorough history and physical examination are crucial in determining the underlying cause of macrocytic anaemia.

**Correct Answer:** C.