Which condition involves swelling due to fluid retention?

The condition that involves swelling due to fluid retention is edema. Edema occurs when excess fluid builds up in the body's tissues, which can lead to visible swelling in areas such as the feet, ankles, legs, or even the lungs. This accumulation of fluid can be caused by various factors, including heart, kidney, or liver problems, and is often a sign that the body is struggling to manage its fluid balance properly. Understanding edema is crucial for a combat medic, as it can indicate underlying health issues that require medical attention.

In contrast, pneumothorax is related to air trapping in the pleural space and does not involve fluid retention. Emphysema is a chronic lung condition that results in damage to the alveoli and does not specifically relate to fluid retention. Fibrosis is the thickening and scarring of connective tissue, which can affect organ function but is not characterized by fluid accumulation in tissues. Thus, edema is distinctly recognized by the presence of swelling due to fluid retention.