Correct Answer: B

Rationale: Morphine is a potent opioid analgesic that primarily acts by binding to μ-opioid receptors in the central nervous system (CNS). This binding alters the perception of and emotional response to pain, resulting in significant analgesia. The scenario describes a rapid reduction in pain intensity from 8/10 to 3/10, which corresponds with the expected onset of Morphine’s effects. The drug does not work by inhibiting prostaglandin synthesis (as NSAIDs do) or altering dopamine or serotonin receptor activity in the same way as antidepressants or antipsychotics. The observed clinical outcome—reduced pain—strongly supports that activation of CNS opioid receptors is the mechanism responsible.

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