A nurse is assessing a client who has a history of multiple substance use disorders. The client is currently experiencing anger, difficulty concentrating, and increased appetite. The nurse should suspect the client is withdrawing from which of the following substances?

Nicotine withdrawal produces a characteristic cluster of symptoms that includes irritability and anger, difficulty concentrating, increased appetite and weight gain, restlessness, depressed mood, anxiety, and strong cravings. The presentation described — anger, difficulty concentrating, and increased appetite — is consistent with the DSM criteria for nicotine withdrawal. Inhalant withdrawal is not well-characterized and does not prominently feature increased appetite or difficulty concentrating as hallmark features. Heroin (opioid) withdrawal typically presents with flu-like symptoms including lacrimation, rhinorrhea, yawning, piloerection, diaphoresis, muscle aches, nausea, vomiting, and diarrhea — not increased appetite or difficulty concentrating as predominant features. Phencyclidine (PCP) withdrawal is not widely recognized as a formal withdrawal syndrome; PCP intoxication, rather than withdrawal, is associated with agitation, psychosis, and violence. The specific combination of anger, concentration difficulty, and increased appetite most precisely maps to the well-established nicotine withdrawal syndrome, making nicotine the correct answer. Increased appetite is particularly distinctive for nicotine withdrawal because nicotine normally suppresses appetite, so its absence leads to increased hunger.