What sign is present?
Behind the cardiac shadow, a sharp change in density is consistent with "sail sign" - left lower lobe collapse.
What would you recommend?
Repeat imaging following treatment (with antibiotics/physiotherapy) is recommended. Depending on local policy, follow up CXR to ensure resolution or CT to excluding an obstructing mass.
Airspace opacification in the left lower zone, with loss of definition of the medial left hemidiaphragm. With this, a straight border is seen going from the diaphragm to the AP window. This is known as the "sail sign" and demonstrates left lower lobe collapse.