**Core Concept:** Pethidine and morphine are opioid analgesics, both acting as competitive antagonists at the µ-opioid receptors in the central nervous system (CNS) and gastrointestinal tract, producing analgesia, sedation, and respiratory depression. However, they differ in their pharmacokinetics and side effect profiles.

**Why the Correct Answer is Right:** Pethidine, also known as meperidine, has a faster onset and shorter duration of action compared to morphine. This is due to its higher lipophilicity, allowing for faster absorption and distribution across the blood-brain barrier. The shorter duration of action is attributed to its rapid hydrolysis by the enzyme monoamine oxidase (MAO) into norpethidine, which is rapidly cleared from the body.

**Why Each Wrong Option is Incorrect:**

A. Morphine is less lipophilic than pethidine, leading to slower absorption and distribution. However, morphine is also broken down by MAO into 6-monoacetylmorphine, which is less lipophilic than morphine itself and can contribute to the slower clearance of morphine from the body.

B. This option is incorrect because morphine has a similar onset time to pethidine.

C. Morphine has a longer duration of action than pethidine due to its less rapid hydrolysis by MAO. However, morphine is also less lipophilic than pethidine, which contributes to its slower clearance.

D. This option is incorrect because pethidine's faster clearance time does not lead to a greater side effect profile compared to morphine. In fact, morphine has a lower incidence of respiratory depression compared to pethidine due to its slower clearance.

**Clinical Pearl:** The faster clearance of pethidine contributes to its higher incidence of side effects such as respiratory depression, nausea, and vomiting. Morphine is generally preferred for pain relief in clinical practice due to its more favorable side effect profile.