Which medical specialty specializes in the treatment of adult illnesses?

Internal medicine is the correct answer because it specifically focuses on the diagnosis, treatment, and prevention of diseases in adults. Physicians specialized in internal medicine, known as internists, typically have extensive training in managing complex medical conditions that affect various organ systems in adult patients. They are equipped to handle a wide range of illnesses and are often the primary care physicians for adult patients, managing chronic diseases like diabetes, hypertension, and heart disease.

In contrast, ophthalmology is concerned with eye disorders and vision care, oncology focuses on the diagnosis and treatment of cancer, and toxicology deals with the effects of poisons and toxic substances. While all these specialties are crucial in their respective fields, they do not center broadly on adult illnesses in the way that internal medicine does.