Which of the following symptoms should heart failure patients learn to recognize as warning signs?

Shortness of breath with usual activities is a significant warning sign for heart failure patients as it can indicate worsening heart function or fluid overload. When the heart's ability to pump blood effectively declines, it can lead to the accumulation of fluids in the lungs, causing symptoms like dyspnea, particularly during exertion or even at rest.

Recognizing this symptom is crucial for patients because it prompts timely communication with healthcare providers, potentially leading to early interventions. Early recognition can prevent further complications, hospitalizations, or exacerbations of heart failure.

In contrast, the other options—such as increased energy levels, decreased appetite, and improved mobility—do not typically signify deterioration in a heart failure patient's condition. Instead, they may suggest stability or even improvement in their clinical status, which is not what the question is focusing on regarding warning signs.