**Core Concept**
Rheumatic heart disease (RHD) is a chronic inflammatory condition that can lead to valvular heart disease, particularly mitral stenosis. Postpartum hemorrhage (PPH) is a life-threatening condition requiring immediate management. In this scenario, the patient's underlying RHD and the need for PPH management intersect, necessitating caution with certain medications.

**Why the Correct Answer is Right**
In patients with RHD, especially those with mitral stenosis, the use of ergot alkaloids like ergometrine (or methylergometrine) is contraindicated. Ergot alkaloids can cause intense vasoconstriction, which can exacerbate the already compromised cardiac output in patients with RHD. This can lead to severe hypertension, myocardial ischemia, and potentially life-threatening complications. Ergometrine is often used to manage PPH, but its use in patients with RHD requires careful consideration.

**Why Each Wrong Option is Incorrect**
**Option A:** Oxytocin is actually a recommended treatment for PPH and is safe to use in patients with RHD. It is a vasopressin analogue that stimulates uterine contractions without causing significant vasoconstriction.

**Option B:** Carboprost tromethamine (15-methyl-PGF2α) is a prostaglandin analogue used to manage PPH. While it can cause vasoconstriction, it is not contraindicated in patients with RHD and may even be beneficial in reducing uterine bleeding.

**Option C:** Methylergometrine is the correct answer, as mentioned earlier. It is an ergot alkaloid that should be avoided in patients with RHD due to its potential to exacerbate cardiac complications.

**Clinical Pearl / High-Yield Fact**
When managing PPH in patients with RHD, it is essential to consider the potential interactions between the patient's underlying cardiac condition and the medications used to manage the hemorrhage. Ergot alkaloids like ergometrine should be avoided, and alternative treatments like oxytocin or carboprost tromethamine should be considered.

**Correct Answer: C. Methylergometrine is contraindicated in patients with rheumatic heart disease and postpartum hemorrhage.**