A 5-year-old girl is brought to the ENT clinic with a 2-week history of intermittent cough, low-grade fever, and recurrent left-sided pneumonia on chest X-ray. Her mother reports that she had choked on a small toy 3 weeks ago but seemed to recover. Flexible laryngoscopy shows no obstruction in the larynx. What is the most likely location of the foreign body, and what is the next step in diagnosis?