Malignancy (Lung Cancer): Given the patient's age, symptoms of dry cough, shortness of breath, inability to lie flat (orthopnea), significant weight loss over a short period, and the presence of a right-sided pleural effusion with atelectasis on CT scan, lung cancer is a highly probable diagnosis. The pleural effusion and atelectasis can be direct consequences of the tumor obstructing airways or invading the pleura.

Pulmonary Embolism: Although less likely than malignancy given the chronic nature of symptoms and weight loss, pulmonary embolism can cause sudden onset of shortness of breath and can be associated with pleural effusion if there is infarction of the lung tissue. It's a consideration, especially in elderly patients with possible decreased mobility.
Chronic Heart Failure: This condition can lead to pleural effusions, especially if the patient has a history of heart disease. The symptoms of shortness of breath, orthopnea, and weight loss could be consistent with heart failure, although the dry cough and significant weight loss might be less typical.
Pneumonia or Empyema: An infectious process could explain the pleural effusion and atelectasis, especially if the patient has been unwell for a month. However, the chronic nature of symptoms and significant weight loss might suggest a more chronic condition.

Tuberculosis (TB): Although less common in some regions, TB can present with chronic cough, weight loss, and pleural effusion. It's crucial to consider TB, especially if the patient has risk factors such as immunocompromised status or exposure history.
Pulmonary Vasculitis (e.g., Wegener's Granulomatosis): These conditions can cause a wide range of pulmonary symptoms, including cough, shortness of breath, and pleural effusions. They are less common but can be life-threatening if not diagnosed and treated promptly.

Lymphangitic Carcinomatosis: A rare condition where cancer spreads to the lymphatic vessels of the lungs, causing symptoms similar to those presented by the patient.
Mesothelioma: A rare form of cancer that affects the lining of the lungs (pleura) and can cause pleural effusions and atelectasis. It's associated with asbestos exposure and has a long latency period.
Sarcoidosis: An autoimmune disease that can affect the lungs and cause a variety of symptoms, including cough, shortness of breath, and pleural effusions. It's less likely given the patient's age and the specific combination of symptoms.

Detailed History and Physical Examination: Including questions about exposure to asbestos, smoking history, and any systemic symptoms.
Imaging: The CT scan has already been done, but considering a PET scan for further evaluation of the pleural effusion and potential metastatic disease if malignancy is suspected.
Pleural Fluid Analysis: Thoracentesis to analyze the pleural fluid for cytology, microbiology, and biochemical analysis (e.g., ADA for TB, glucose levels).
Biopsy: If malignancy is suspected, a tissue diagnosis is crucial. This could involve a CT-guided biopsy of the lung lesion or a pleural biopsy.
Cardiac Evaluation: To rule out heart failure, including echocardiography.
Infectious Disease Workup: Depending on the clinical suspicion, including tests for TB and other infections.