**Core Concept**
Pethidine, also known as meperidine, is a synthetic opioid analgesic that undergoes extensive hepatic metabolism. Its long-term use can lead to the accumulation of a toxic metabolite, which is responsible for various adverse effects.

**Why the Correct Answer is Right**
The primary concern with long-term pethidine use is the accumulation of normeperidine, a metabolite that is associated with central nervous system (CNS) toxicity. Normeperidine is an active metabolite that can cause seizures, tremors, and myoclonus due to its ability to inhibit the enzyme gamma-aminobutyric acid transaminase (GABA-T), leading to an accumulation of inhibitory neurotransmitter GABA in the CNS. This results in an imbalance of neurotransmitter activity, contributing to the development of CNS toxicity.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while pethidine is associated with various side effects, such as respiratory depression and constipation, the primary concern with long-term use is not related to these effects.

**Option B:** This option is incorrect because pethidine is not primarily metabolized by the liver enzyme CYP2D6, which is involved in the metabolism of other opioids like codeine and tramadol.

**Option C:** This option is incorrect because while pethidine can cause dependence and addiction, the primary concern with long-term use is not related to its addictive potential, but rather the accumulation of a toxic metabolite.

**Clinical Pearl / High-Yield Fact**
To avoid CNS toxicity associated with long-term pethidine use, it is essential to monitor patients for signs of neurological impairment, such as tremors, seizures, and myoclonus, and consider alternative analgesics that do not have the same risk of metabolite accumulation.

**Correct Answer: C. Normeperidine.