A 55-year-old woman from Hong Kong with a 2-month history of progressive right nasal obstruction and right-sided neck swelling presents to the ENT clinic. She reports no smoking or alcohol use. Flexible nasopharyngoscopy shows a friable, bleeding mass in the right nasopharynx. CT imaging reveals a 4 cm nasopharyngeal mass with erosion into the clivus and right cervical lymph nodes (largest 3 cm). Biopsy confirms squamous cell carcinoma. Which of the following is the strongest independent prognostic factor for this patient's overall survival?