Silhouette Sign Middle Lobe Pneumonia

A 55-year-old woman presents with 4 days of right-sided pleuritic chest pain, productive cough with rusty-coloured sputum, and high-grade fever. Examination reveals decreased expansion on the right, dull percussion over the right anterior mid-zone, and bronchial breath sounds. A frontal chest radiograph shows homogeneous consolidation with air bronchograms. The structure marked **A** demonstrates obliteration of the right heart border with a preserved sharp right hemidiaphragm. Based on the silhouette sign, which of the following is the most likely diagnosis?