Lung Cancer: The presence of a large apical mass, particularly in the context of a small pleural effusion, strongly suggests lung cancer, especially non-small cell lung cancer (NSCLC) or small cell lung cancer (SCLC). The apical location is classic for Pancoast tumors, a subtype of NSCLC.

Tuberculosis (TB): TB can present with a large apical mass due to caseation and necrosis, along with a small pleural effusion. It's a common cause of such findings, especially in endemic areas or in immunocompromised patients.
Pneumonia with Complications: Complicated pneumonia, especially due to Staphylococcus aureus or Klebsiella pneumoniae, can lead to the formation of an apical mass (e.g., lung abscess) and a small pleural effusion.
Pulmonary Metastasis: Metastatic disease to the lung from other primary sites (e.g., breast, colon, or melanoma) can present with multiple masses, but a solitary large apical mass with a small pleural effusion is also possible.

Pulmonary Embolism with Infarction: Although less common, a large pulmonary embolism can cause infarction of the lung tissue, leading to a mass-like appearance on imaging, accompanied by a small pleural effusion. Missing this diagnosis could be fatal.
Empyema: An empyema, or pus in the pleural space, can sometimes present with a mass-like appearance on the chest wall or mediastinum due to the inflammatory process, along with a small to moderate pleural effusion.

Wegener's Granulomatosis (Granulomatosis with Polyangiitis, GPA): This autoimmune disease can cause necrotizing granulomatous inflammation of the respiratory tract, leading to masses and pleural effusions, though it's less common.
Lymphoma: Both Hodgkin and non-Hodgkin lymphoma can involve the lung and cause masses, but this is less common than other diagnoses listed here.
Hydatid Disease: In endemic areas, hydatid cysts caused by Echinococcus granulosus can present as lung masses, sometimes with pleural effusion, though this is relatively rare.