**Core Concept**
Etomidate and thiopentone are barbiturates, while ketamine is a dissociative anesthetic. However, the question focuses on pain on injection, a common side effect of certain anesthetic agents. Propofol is a phenolic compound known for causing pain on injection due to its lipid nature and high pH.

**Why the Correct Answer is Right**
Propofol's pain on injection is attributed to the presence of free fatty acids, which cause irritation and inflammation at the injection site. This is due to the low pH and the presence of propofol's lipid emulsion, which can cause histamine release and vasodilation. The pain can be mitigated by using a larger vein, warming the propofol, or administering a lidocaine bolus before the propofol injection.

**Why Each Wrong Option is Incorrect**
**Option A (Etomidate):** Etomidate is known for causing adrenal suppression, but it is not typically associated with pain on injection. Its low incidence of side effects makes it a popular choice for induction in patients with cardiovascular instability.
**Option B (Methohexitone):** Methohexitone is a barbiturate with a rapid onset and short duration, but it is not commonly reported to cause pain on injection. It is often used for procedural sedation and day-case anesthesia.
**Option C (Ketamine):** Ketamine is a dissociative anesthetic that can cause hallucinations and emergence reactions, but it is not typically associated with pain on injection. It can actually be used to reduce pain perception and is often used for procedural sedation.
**Option D (Thiopentone):** Thiopentone is a barbiturate with a rapid onset and short duration, but it is not commonly reported to cause pain on injection. It is often used for induction in emergency situations and is known for its rapid action.

**Clinical Pearl / High-Yield Fact**
When administering propofol, always ensure that the patient is comfortable and the vein is large enough to accommodate the lipid emulsion. Warming the propofol and administering a lidocaine bolus can help reduce the incidence of pain on injection.

**Correct Answer:** D. Thiopentone