Final answer:
D) Morphine is the CNS drug that acts on the same receptor as the endogenous peptide, providing pain relief, euphoria, and sedation by targeting the μ-opioid receptor.
Step-by-step explanation:
The endogenous peptide described in the question acts on receptors within the central nervous system (CNS) to provide pain relief as well as euphoria and sedation. Based on the effects described, such as pain relief and euphoria, and considering the listed options, the CNS drug that acts on the same receptor is most likely morphine. Morphine is a well-known opioid that binds to and activates the μ-opioid receptor, leading to analgesic, euphoric, and sedative effects. Isoflurane is an inhalational anaesthetic, succinylcholine is a neuromuscular blocker, lithium is used in mood disorders, and scopolamine has anticholinergic properties—none of which specifically target the opioid receptors responsible for the effects described.