Correct Answer: A

Rationale: Morphine, a potent opioid analgesic, exerts its effects primarily through activation of μ-opioid receptors in the central nervous system. This action inhibits ascending pain pathways, alters pain perception, and increases pain tolerance. The sharp reduction in pain score following administration indicates successful receptor activation and analgesia. Morphine does not act via prostaglandin inhibition like NSAIDs or serotonin modulation like SSRIs, confirming that its primary mechanism involves opioid receptor-mediated suppression of nociceptive transmission.

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