**Core Concept**
Macrocytic anaemia is a type of anaemia characterized by large red blood cells (macrocytosis). It occurs due to impaired DNA synthesis, leading to increased cell cycle duration, which results in the production of larger than normal red blood cells. This can be caused by various factors, including deficiencies in vitamin B12 or folate, which are essential for DNA synthesis.

**Why the Correct Answer is Right**
The correct answer is related to the causes of macrocytic anaemia. Macrocytic anaemia typically occurs due to impaired DNA synthesis, often caused by deficiencies in vitamin B12 or folate. These deficiencies lead to the accumulation of homocysteine and methylmalonic acid, resulting in the production of macrocytic red blood cells. The key enzyme responsible for this process is methionine synthase, which requires vitamin B12 as a cofactor for its activity.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because macrocytic anaemia can indeed occur due to a deficiency in vitamin B12, which is a well-established cause of this condition.
* **Option B:** This option is incorrect because folate deficiency is another known cause of macrocytic anaemia, leading to impaired DNA synthesis and the production of larger red blood cells.
* **Option C:** This option is incorrect because hypothyroidism can indirectly lead to macrocytic anaemia by causing a decrease in the production of erythropoietin, which can result in impaired red blood cell production.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that macrocytic anaemia is often associated with elevated levels of homocysteine and methylmalonic acid, which can be used as diagnostic markers for this condition.

**Correct Answer:** D