What finding is most indicative of opioid intoxication in a patient with altered mental status and track marks on his arms?

B. Miosis is the most indicative finding of opioid intoxication. Opioids, such as heroin or prescription pain medications, typically cause constricted pupils, known as miosis. This pinpoint pupil reaction is a classic sign associated with opioid effects on the central nervous system.

In addition to altered mental status, the presence of track marks suggests intravenous drug use, often associated with opioid consumption. The combination of these symptoms and the characteristic miosis further solidify the diagnosis of opioid intoxication.

Other findings that might be present during opioid intoxication include respiratory depression and sedation; however, the hallmark pupil finding of miosis in the context given makes it the most definitive indicator in this scenario. Elevated temperature, mydriasis (dilated pupils), and tremors may occur with other conditions or substances and are not specific to opioid intoxication.