1) Most likely diagnosis and why: Left pleural effusion with passive left lower‑lobe atelectasis — demonstrated by blunting/meniscus at the left costophrenic sulcus and dependent layering of fluid on the upright film. 2) Next best diagnostic step: Thoracic ultrasound (or left lateral decubitus radiograph if US unavailable) to confirm a free‑flowing effusion, estimate volume and guide safe thoracentesis. 3) Key differential or confirmatory test: Diagnostic thoracentesis with pleural fluid analysis (cell count, protein, LDH for Light’s criteria, Gram stain/culture, cytology) to differentiate transudate vs exudate and identify infection or malignancy; consider chest CT if loculation or underlying intrathoracic pathology is suspected. 4) Management: therapeutic thoracentesis for symptomatic relief and diagnosis; if empyema then chest tube drainage + targeted antibiotics; recurrent malignant effusion → indwelling pleural catheter or pleurodesis; treat underlying cause (e.g., heart failure, infection, malignancy).