Which of the following glands is not part of the endocrine system?

The spleen is not part of the endocrine system; instead, it plays a vital role in the lymphatic system and in filtering blood. The endocrine system consists of glands that produce hormones, which are critical for regulating various bodily functions. In contrast, the spleen's primary functions include filtering blood, recycling iron from red blood cells, and aiding in immune responses. The other glands mentioned, such as the thyroid, adrenal, and thymus glands, all secrete hormones important for metabolism, stress response, and immune system regulation, respectively. Thus, the correct answer clearly highlights the distinction between the roles of the spleen and those of the glands in the endocrine system.