There is still much debate to whether<unk>  is a congenital problem or acquired through recurrent pulmonary infection. It is widely believed that extralobar<unk> s are a result of prenatal pulmonary malformation while intralobar<unk> s can develop due to recurrent pulmonary infections in adolescents and young adults. The most frequently supported theory of<unk>  formation involves an accessory lung bud that develops from the ventral aspect of the primitive foregut. The pluripotential tissue from this additional lung bud migrates in a caudal direction with the normally developing lung. It receives its blood supply from vessels that connect to the aorta and cover the primitive foregut. These attachments to the aorta remain to form the systemic arterial supply of the<unk> . Early embryologic development of the accessory lung bud results in formation of the<unk>  within normal lung tissue. The<unk>  is encased within the same pleural covering. This is the intrapulmonary variant. In contrast, later development of the accessory lung bud results in the extrapulmonary type that may give rise to communication with the GI tract. Both types of<unk>  usually have arterial supply from the thoracic or abdominal aorta. Rarely, the celiac axis, internal mammary, subclavian, or renal artery may be involved. Intrapulmonary<unk>  occurs within the visceral pleura of normal lung tissue. Usually, no communication with the tracheobronchial tree occurs. The most common location is in the posterior basal segment, and nearly two thirds of<unk> s appear in the left lung. Venous drainage is usually via the pulmonary veins. Foregut communication is very rare, and associated anomalies are uncommon. Extrapulmonary<unk>  is completely enclosed in its own pleural sac. It may occur above, within, or below the diaphragm, and nearly all appear on the left side. No communication with the tracheobronchial tree occurs. Venous drainage is usually via the systemic venous system. Foregut communication and associated anomalies, such as diaphragmatic hernia, are more common.
Pulmonary sequestration
Cryptogenic organizing pneumonia
Lung abscess
Rheumatoid pleuritis
Atypical pneumonia
Lobar pneumonia
Malignant pleural effusion
Pulmonary alveolar proteinosis