A 6-yr-old girl presents 72 hr after undergoing a tonsillectomy/adenoidectomy with fever, cough, dyspnea, and right-sided chest pain. Examination reveals an ill-appearing child; her temperature is 104°F, pulse rate 110/min, and respiratory rate 40/min; her lungs have decreased air movement on the right, with dullness to percussion. Complete blood count demonstrates marked leukocytosis with left shift, and appearance on chest radiograph is consistent with a pulmonary abscess.Which of the following is the most appropriate initial management?