What is a potential outcome of decreased testosterone levels?

Decreased testosterone levels can lead to reduced libido, which refers to a diminished interest in sexual activity and desire. Testosterone is a crucial hormone that helps regulate sexual function and other aspects of men's health. When testosterone levels drop, it can directly affect libido, leading to a noticeable decrease in sexual desire.

This outcome is particularly relevant because testosterone influences not only sexual health but also emotional and psychological well-being. A decline in testosterone can result in various physical and emotional changes, contributing to a complex interplay of symptoms that might affect overall quality of life.

The other choices illustrate different aspects of testosterone's role in the body. For instance, increased muscle mass and development of secondary sexual characteristics generally occur with adequate testosterone levels, while increased body fat can also be associated with low testosterone; however, the primary and most immediate effect among these is reduced libido.