What substance lubricates the skin or scalp and is secreted by sebaceous glands?

Sebum is the correct answer because it is the oily substance produced by the sebaceous glands that plays a crucial role in maintaining skin and scalp health. This lubricating oil helps to keep the skin hydrated by preventing moisture loss, which is essential for maintaining skin elasticity and overall appearance. Additionally, sebum possesses antibacterial properties that protect the skin from certain infections and helps to create a barrier against environmental pollutants.

Cholesterol is a type of lipid that is found in cell membranes and is important for various bodily functions, but it is not secreted by sebaceous glands. Keratin is a protein that forms the structural material for hair, nails, and the outer layer of skin, contributing to durability and resilience rather than lubrication. Collagen is a protein that provides structure and elasticity to the skin, but like keratin, it is not a lubricating substance produced by sebaceous glands.