A 68-year-old woman presents to her primary care physician with a 6-month history of progressive difficulty swallowing, initially with solid foods, now also with liquids. She reports an unintentional weight loss of 8 kg (17.6 lbs) over the past 4 months, accompanied by persistent fatigue and a nocturnal cough. For several years, she has experienced chronic heartburn and regurgitation, often requiring daily antacid use for mild relief. Her medical history includes well-controlled type 2 diabetes mellitus and gastroesophageal reflux disease. She has a 20-pack-year smoking history, having quit 15 years ago, and consumes alcohol socially (1-2 glasses of wine per week). On examination, her BMI is 31 kg/m^2. Vital signs are stable with a temperature of 36.8°C (98.2°F), blood pressure of 138/88 mmHg, pulse of 76/min, and respirations of 14/min. Oxygen saturation is 97% on room air. She appears pale with mild temporal wasting. Initial evaluation at an outpatient facility revealed a suspicious lower esophageal mass during an esophagogastroduodenoscopy (EGD). Which of the following represents the most likely diagnosis and the critical next step in management for this patient?

A 72-year-old woman presents to her primary care physician's office with increasing difficulty swallowing and a persistent feeling of food sticking in her throat. Her symptoms, initially limited to solid foods, have slowly worsened over the past eight months, and she now struggles to consume even pureed liquids. She reports significant unintentional weight loss of approximately 9 kg (20 lbs) over the same period and experiences occasional regurgitation of undigested food. Her medical history includes well-controlled hypertension and a remote history of heavy alcohol consumption for 20 years, ceasing 15 years ago, alongside a 40 pack-year smoking history. Physical examination reveals temporal wasting and evidence of diffuse muscle atrophy. There is a palpable, non-tender supraclavicular lymph node on the left side. Given these findings, what is the most likely diagnosis and the appropriate initial diagnostic steps to confirm it?