Which of the following is an aromatase inhibitor used in breast cancer treatment?

Letrozole is recognized as an aromatase inhibitor used in the treatment of breast cancer, particularly hormone receptor-positive breast cancer in postmenopausal women. Aromatase inhibitors work by blocking the aromatase enzyme, which converts androgens into estrogens. By reducing estrogen levels, these medications help to limit the growth of estrogen-sensitive tumors.

Letrozole specifically is effective in reducing the risk of cancer recurrence and is often used after surgery or as a first-line treatment option. In contrast, other medications listed, such as fulvestrant, are not classified as aromatase inhibitors; fulvestrant serves as an estrogen receptor antagonist. Imatinib is primarily used for leukemia and gastrointestinal stromal tumors, while Opdivo, known generically as nivolumab, is an immunotherapy drug targeted for various types of cancer, including melanoma and lung cancer. These distinctions clarify why letrozole stands out as the correct answer in this context.