A 52-year-old male with a history of ankylosing spondylitis presents for elective lumbar spine fusion under general anesthesia. On airway assessment, he has a Mallampati score of III, limited neck extension due to fixed kyphosis, and a thyromental distance of 5 cm. After induction with propofol and succinylcholine, the anesthesiologist attempts direct laryngoscopy but visualizes only the epiglottis (Cormack-Lehane Grade 3). Bag-mask ventilation is adequate. What is the most appropriate next step according to the difficult airway algorithm?