Which condition would most likely be treated with activated charcoal?

Activated charcoal is commonly used in certain cases of poisoning or overdose to prevent the absorption of toxins from the gastrointestinal tract into the bloodstream. In the context of an antidepressant overdose, activated charcoal can bind to the antidepressant medication and significantly reduce its bioavailability if administered soon after the ingestion of the drug. This treatment helps mitigate the toxic effects of the overdose, making it a standard intervention in emergency settings for specific types of drug overdoses.

In contrast, pneumonia, appendicitis, and diabetes are conditions that do not involve toxicity from ingesting substances that activated charcoal can neutralize. Pneumonia is an infection that requires antibiotics or supportive care; appendicitis presents a need for surgical intervention; and diabetes management centers around monitoring blood glucose levels and administering insulin or other medications, rather than dealing with poison absorption. Therefore, the use of activated charcoal is specifically indicated for overdoses, with antidepressant overdose being a prime example where it is effective.