**Core Concept**
Ergometrine is an oxytocic medication used to prevent or treat postpartum hemorrhage by causing sustained uterine contractions. It works by stimulating the alpha-adrenergic receptors in the smooth muscle of the uterus, leading to increased uterine tone and reduced uterine blood flow.

**Why the Correct Answer is Right**
The primary contraindication of ergometrine is heart disease, particularly conditions that affect the cardiovascular system such as coronary artery disease, heart failure, or arrhythmias. This is because ergometrine can cause vasoconstriction and increase blood pressure, which may exacerbate existing cardiovascular conditions. Additionally, ergometrine can also lead to myocardial ischemia or infarction in susceptible individuals.

**Why Each Wrong Option is Incorrect**
**Option A:** Diabetes mellitus is not a contraindication for ergometrine, although patients with diabetes may be more prone to hypoglycemia due to the medication's potential to cause hypoglycemia.

**Option B:** Excessive post-partum hemorrhage is actually an indication for ergometrine, as it helps to control bleeding and promote uterine contractions.

**Option D:** Anaemia is not a contraindication for ergometrine, although patients with anaemia may be more susceptible to the medication's effects on blood pressure and cardiac output.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that ergometrine can cause hypertension and cardiac complications in susceptible individuals, making heart disease a critical contraindication. Always assess the patient's cardiovascular status before administering ergometrine.

**✓ Correct Answer: C. Heart disease**