Which symptom is specifically characterized as orthopnea in patients with CHF?

Orthopnea is specifically defined as a type of dyspnea that occurs when a person is lying flat. In patients with congestive heart failure (CHF), this symptom arises due to the redistribution of body fluids when supine, which can lead to pulmonary congestion and increased pressure in the lungs. This phenomenon makes it difficult for individuals to breathe comfortably while lying down, prompting them to seek relief by propping themselves up or sleeping in a more upright position.

Although the other options describe symptoms related to respiratory distress, none of them capture the specific condition of orthopnea. Difficulty breathing during physical activity relates to exertional dyspnea, sudden shortness of breath at night pertains to paroxysmal nocturnal dyspnea, and constant fatigue and weakness can be general symptoms associated with heart failure but do not specifically denote breathing difficulties in a particular position. Therefore, the correct answer accurately reflects the defining feature of orthopnea in the context of CHF.