<s>[INST] context: This chest x-ray shows right upper zone opacity with multiple air-fluid levels. Surrounding this opacity and the air-fluid levels is an ill-defined infiltrate. Note that the right apex and lower lung zones are clear. The left lung is also clear, and no pleural or mediastinal disease is noted. This x-ray is consistent with a necrotizing process in the posterior segment of the right upper lobe, with an air-fluid level such as in a lung abscess. The presence of a subacute illness with foul-smelling sputum and clubbing is a classical presentation for a lung abscess, which is caused by mixed aerobic and anaerobic infection. Pneumococcal pneumonia generally does not produce a necrotizing picture, and PCP in an immune-compromised patient usually appears as a diffuse reticular pattern, but cystic and cavitary areas may be present. A lung abscess represents a localized area of lung necrosis with a thick wall and an acute angle to the chest wall. An air-fluid level is seen, representing a communication with the bronchial tree. The fluid level diameter is usually similar on PA and lateral chest films with a lung abscess and is often considerably different with empyema. Differentiation between an empyema and a lung abscess is crucial since the former requires tube thoracostomy, while the latter is adequately treated with antibiotics and postural drainage. Immediate bronchoscopy may not be needed unless an endobronchial obstruction is suspected or there is a history of choking or aspiration. Contrast CT scans may be helpful to see the split pleural sign with marked visceral and parietal pleural thickening in an empyema. [/INST] question: A 40-year-old man with a history of substance abuse and HIV infection is seen in the ER with complaints of fever, weight loss, production of foulsmelling sputum, and shoness of breath for 2 wk. On physical exam he is tachypneic and has clubbing of his digits. Lung exam reveals diffuse rhonchi and an area of egophony with whispering pectoriloquy in the right chest posteriorly. ABGs reveal PaO2 of 59 mm Hg on room air. CXR is shown.What is the most likely diagnosis? | options: A: Pneumococcal pneumonia B: PCP pneumonia C: Lung abscess D: Squamous cell carcinoma | answer: C </s>