Heart Failure: The most common cause of pleural effusion in patients with HFrEF (Heart Failure with Reduced Ejection Fraction) is the heart failure itself. The increased pressure in the pulmonary veins can lead to fluid leakage into the pleural space, and the right side can be affected due to the systemic venous congestion that often accompanies heart failure.

Pulmonary Embolism: Although less common than heart failure as a cause, pulmonary embolism can lead to pleural effusion, especially if it is large or involves the right lung. The effusion is usually exudative and can be accompanied by other symptoms such as chest pain and dyspnea.
Pneumonia: Bacterial or viral pneumonia can cause a pleural effusion, and in the context of heart failure, the patient might be more susceptible to infections due to decreased mobility, aspiration, or other factors.
Liver Disease: In patients with heart failure, especially those with a history of alcohol abuse or other liver conditions, liver disease can lead to fluid retention and pleural effusions due to hypoalbuminemia and portal hypertension.

Malignancy: Although less likely, malignancies such as lung cancer or metastatic disease can cause pleural effusions. Given the potential for serious outcomes if missed, it's crucial to consider and rule out malignancy, especially in patients with risk factors or unexplained weight loss.
Tuberculosis: TB can cause pleural effusions, and while less common in some regions, it remains a critical diagnosis to consider due to its treatability and the consequences of missing the diagnosis.
Pulmonary Arterial Hypertension: This condition can worsen heart failure symptoms and lead to pleural effusions. It's a critical diagnosis to consider because it significantly affects management and prognosis.

Amyloidosis: Systemic amyloidosis can involve the heart and lead to heart failure, and it can also cause pleural effusions. It's a rare condition but should be considered in patients with unexplained heart failure and pleural effusion without a clear cause.
Sarcoidosis: This autoimmune disease can affect the lungs and heart, leading to pleural effusions and heart failure. It's less common but should be considered in the differential diagnosis, especially in younger patients or those with other systemic symptoms.
Lymphangitic Carcinomatosis: This rare condition involves the spread of cancer to the lymphatic vessels of the lung, leading to pleural effusions. It's an important diagnosis to consider in patients with known cancer and pleural effusions.