**Core Concept**
Diacetyl morphine, also known as heroin, is a semi-synthetic opioid derived from morphine. It is a highly addictive substance due to its rapid onset of action, which leads to intense euphoria. Heroin's abuse potential is significantly higher than other opioids due to its rapid absorption through mucous membranes, bypassing the liver's first-pass metabolism.

**Why the Correct Answer is Right**
Heroin's high abuse potential is attributed to its rapid onset of action, which is mediated by the mu-opioid receptors in the brain. Unlike other opioids, heroin is not subject to the same degree of first-pass metabolism in the liver, allowing it to reach the brain quickly and produce intense euphoria. This rapid absorption through mucous membranes further contributes to its high abuse potential.

**Why Each Wrong Option is Incorrect**
**Option A:** Morphine is a naturally occurring opioid that is commonly used for pain management, but it is not as rapidly absorbed as heroin and does not have the same high abuse potential.
**Option C:** Codeine is a naturally occurring opioid that is often used as a cough suppressant and mild analgesic. It has a lower abuse potential compared to heroin due to its slower onset of action and lower potency.
**Option D:** Fentanyl is a synthetic opioid that is highly potent and rapidly absorbed, but it is typically used in medical settings for pain management and anesthesia. Its high potency and rapid absorption make it a serious concern for overdose, but it is not as commonly abused as heroin.

**Clinical Pearl / High-Yield Fact**
Heroin's rapid onset of action is due to its ability to bypass the liver's first-pass metabolism, which is a critical factor in its high abuse potential. This is in contrast to other opioids, which are metabolized by the liver before reaching the brain.

**✓ Correct Answer: B. Diacetyl morphine. Heroin.**