What oily substance is secreted by sebaceous glands?

Sebaceous glands are specialized glands in the skin that produce an oily substance known as sebum. Sebum plays a crucial role in maintaining healthy skin and hair by providing lubrication and preventing dryness. It acts as a natural moisturizer, helping to keep the skin flexible and protecting it against environmental elements. The composition of sebum typically includes triglycerides, wax esters, and squalene, among other lipids.

While other substances such as fluid, serum, or mucus can be found in the body, none of these directly refer to the substance produced by sebaceous glands. Fluid is a broad term that doesn't specify the characteristics that define sebum. Serum usually refers to the clear liquid that separates from blood when it clots, not relevant in this context. Mucus is a viscous secretion produced by mucous membranes and has a different role entirely, primarily protecting and lubricating surfaces of the respiratory and digestive tracts. Thus, sebum is the specific oily substance that is secreted by sebaceous glands, clearly identifying it as the correct answer.