**Core Concept**
Ketamine is a medication used for starting and maintaining anesthesia. It has a unique property of being a "preservative of cardiovascular stability," making it an ideal choice for patients with compromised cardiovascular systems, such as those with cyanotic heart disease.

**Why the Correct Answer is Right**
Ketamine works by blocking NMDA receptors in the brain, which helps to reduce the need for respiratory and cardiovascular support during anesthesia. This is particularly beneficial for patients with cyanotic heart disease, as it allows for a more stable hemodynamic profile during the perioperative period. Additionally, ketamine has been shown to increase blood pressure and heart rate, which can help to counteract the hypotension that often occurs in patients with cyanotic heart disease. The combination of these effects makes ketamine an attractive option for anesthesia in this patient population.

**Why Each Wrong Option is Incorrect**
**Option A:** Propofol is a commonly used anesthetic agent, but it can cause significant vasodilation and hypotension, which is a particular concern in patients with cyanotic heart disease.
**Option C:** Thiopentone is a barbiturate that can cause significant myocardial depression and vasodilation, leading to a decrease in blood pressure and cardiac output in patients with compromised cardiovascular systems.
**Option D:** Sevoflurane is a volatile anesthetic that can cause a decrease in blood pressure and cardiac output, particularly in patients with cyanotic heart disease.

**Clinical Pearl / High-Yield Fact**
Ketamine's ability to preserve cardiovascular stability makes it an ideal choice for patients with cyanotic heart disease. However, it's essential to note that ketamine can cause an increase in intracranial pressure, so caution should be exercised when using it in patients with head injuries or other conditions that may increase intracranial pressure.

✓ Correct Answer: B. Ketamine