A 28-year-old tall, lean male presents to the emergency department with acute onset right-sided pleuritic chest pain and dyspnea for 2 hours. He denies trauma or recent illness. Vital signs: HR 102/min, RR 24/min, BP 128/82 mmHg, SpO₂ 94% on room air. Chest examination reveals decreased breath sounds on the right side with hyperresonance to percussion. A frontal chest X-ray is obtained. Which radiological sign on the chest X-ray would be most specific for confirming the diagnosis of pneumothorax?