A 2-year-old girl presented with a history of recurrent chest infections  and persistent wheezing that were not responding to treatment. She  was HIV-negative, there was no history of contact with tuberculosis  and all tuberculosis testing including Gene Xpert MTB/RIF was  negative. Plain frontal chest radiograph demonstrated a well-defined  lucency in the left lower lobe medially, with a curvilinear outline  resembling an inferior accessory lobe and fissure. The right lung  appeared smaller than the left, but the heart was normally positioned  (Fig. 1). 
 Bronchoscopy was performed and demonstrated anterior mid-tracheal compression with pulsation. There was also narrowing  of the opening of the left main bronchus with medial and lateral compression with pulsation. The bronchial configuration was  abnormal on the right side with an absent bronchus intermedius. 
 A contrasted chest computed tomography (CT) scan showed an  'isthmus' of the lung in continuity with the right lung, extending  from the right hemithorax across the midline (between the heart  and descending aorta) into the left hemithorax, consistent with a  horseshoe lung (Fig. 2).
The arterial supply was from the right lung  branches (Fig. 3A) and there was a clearly defined pleural separation  from the left lung (the presumed 'isthmic fissure'). Arterial supply to  the isthmus was via the right lower lobe vessels and venous drainage  appeared to be directly into the right atrium. A small bronchial  branch of the left lower lobe bronchial tree was noted to supply the  isthmus, but this appeared attenuated across the vertebral body and  descending aorta, resulting in selective air-trapping of the isthmic  segment (Fig. B). 
 Even though the plain radiograph demonstrated a possible smaller  right lung, there were no CT features of right (or left) pulmonary  hypoplasia. The pulmonary arteries were of adequate size, if not  slightly plump. The heart was normally positioned on the left. There  were, however, some significant associated venous abnormalities  in the form of a scimitar-type vessel, which ascended from the  right lung base peripherally to join another scimitar-type vessel  descending from union of two superior segmental intervertebral  veins, to enter the superior vena cava as it feeds into the right atrium  (Fig. 3C). The right atrium was noted to be prominent. 
 Other CT scan findings included an absent minor fissure and a bovine variant of the aortic arch with the left common carotid artery  arising together with the brachiocephalic trunk from the aortic arch,  a common variant. This appeared to cause some anterior impression  on the trachea superiorly.
Axial multidetector computed tomography (MDCT) on lung 
window demonstrates an isthmus of basal lung in continuity with the 
right lung, extending across the midline between the heart anteriorly 
and the aorta and vertebral body posteriorly. There is a distinct pleural 
interface between the isthmus and the left lung.