## Core Concept
The problem involves converting a patient from intravenous (IV) Diamorphine to oral Morphine. Diamorphine is a potent opioid agonist, and Morphine is a standard opioid used for pain management, especially in palliative care. The conversion between these opioids requires knowledge of their potency ratios.

## Why the Correct Answer is Right
Diamorphine is approximately 3-4 times more potent than Morphine on a milligram-to-milligram basis when given intravenously. However, when converting from IV Diamorphine to oral Morphine, we must consider the oral bioavailability of Morphine, which is about 20-30%. The generally accepted conversion is that 1 mg of IV Diamorphine is equivalent to 3-4 mg of oral Morphine. Given the patient is on IV Diamorphine 10 mg/day, we use the conversion factor to calculate the equivalent oral Morphine dose.

## Why Each Wrong Option is Incorrect
- **Option A:** This option suggests a dose that is too low, not accounting for the proper conversion ratio between IV Diamorphine and oral Morphine.
- **Option B:** This seems to closely align with the conversion but might not precisely reflect the most accurate conversion ratio used in clinical practice.
- **Option D:** This option suggests a dose higher than necessary based on standard conversion ratios.

## Clinical Pearl / High-Yield Fact
A key point to remember is the "Diamorphine to Morphine" conversion ratio. For every 1 mg of Diamorphine, approximately 3-4 mg of oral Morphine is needed. This conversion is crucial in palliative care for managing chronic pain effectively while minimizing side effects.

## Correct Answer: B. 30 mg.