Which of the following best describes the effect of effusion in the body?

The best description of the effect of effusion in the body is increased fluid buildup. Effusion refers to the excess accumulation of fluid in a particular body cavity, such as a joint space. This buildup can occur due to various reasons, such as inflammation, injury, or disease, leading to swelling in the affected area.

In the context of joint effusion, for example, the increased fluid can cause discomfort, restrict movement, and impact the overall function of the joint. This fluid buildup is often a response to irritation or damage in the surrounding tissues, as the body attempts to protect the injured area.

The other options do not accurately capture the specific effects of effusion. Decreased blood flow generally relates to vascular issues rather than fluid accumulation. Muscle tearing is a separate injury mechanism unrelated to the concept of effusion, and increased mobility contradicts the typical outcome of effusion, which often results in decreased range of motion due to swelling and discomfort.