A 6-month-old infant with known tetralogy of Fallot (TOF) presents with a 'Tet spell'—sudden-onset severe cyanosis, dyspnea, and loss of consciousness. After initial resuscitation with oxygen, knee-chest positioning, and morphine, the episode resolves. The parents ask about the anatomic basis of the cyanosis and whether the child needs surgery. Which investigation is most appropriate to assess the severity of the right ventricular outflow tract obstruction and guide timing of surgical repair?