Which of the following medications is a tricyclic antidepressant that can be used as an adjuvant analgesic?

Amitriptyline is a tricyclic antidepressant that has been found effective in the management of neuropathic pain and other chronic pain conditions. Its mechanism of action involves the inhibition of the reuptake of norepinephrine and serotonin, which can help modulate pain pathways and improve the overall pain experience.

In addition to its analgesic properties, amitriptyline has a sedative effect, which can benefit patients who may also be experiencing insomnia related to pain or other conditions. It's particularly useful in treating conditions like fibromyalgia and other chronic pain syndromes, making it a valuable adjuvant in pain management.

The other medications listed, while also used in pain management, do not fall under the category of tricyclic antidepressants. Gabapentin and pregabalin are anticonvulsants indicated for neuropathic pain, while duloxetine is a serotonin-norepinephrine reuptake inhibitor (SNRI), which, though effective for chronic pain, is not classified as a tricyclic antidepressant.