Differential Diagnosis

• Single most likely diagnosis
  • Left lower lobe pneumonia: The presence of a small left basal pleural effusion and increased retrocardiac density raising suspicion for left lower zone collapse/consolidative change is highly suggestive of a left lower lobe pneumonia. The mild upper lobe venous diversion also supports this diagnosis as it indicates some degree of volume loss or increased pressure in the lower lobe.
• Other Likely diagnoses
  • Pulmonary edema: The findings of a pleural effusion and increased density could also be consistent with pulmonary edema, particularly if there is a history of heart failure or other predisposing conditions.
  • Atelectasis: The suspicion for left lower zone collapse could also suggest atelectasis, which might be due to various causes including obstructive lesions or post-operative changes.
  • Pleurisy: The presence of a pleural effusion could indicate pleurisy, which might be infectious or inflammatory in nature.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Pulmonary embolism: Although less likely given the specific findings, pulmonary embolism can sometimes present with pleural effusions and should always be considered, especially if there are risk factors or suggestive symptoms.
  • Malignancy: A new pleural effusion, especially if associated with other findings like lymphadenopathy or a mass, could indicate malignancy, which would have significant implications for management and prognosis.
• Rare diagnoses
  • Diaphragmatic rupture or eventration: These conditions could potentially cause increased retrocardiac density and might be associated with pleural effusions, though they are less common and typically have specific predisposing factors (e.g., trauma).
  • Congenital anomalies: Certain congenital anomalies affecting the lung or pleura could present with similar findings, though these would be rare and often have other distinguishing features or a relevant patient history.