Q. A 45-year-old male patient presents to the gastroenterology clinic with a longstanding history of heartburn, particularly worsening after meals and when lying down. He also reports occasional regurgitation of acidic content into his mouth, especially at night. Over-the-counter antacids provide temporary relief. He has tried lifestyle modifications, such as weight loss and elevating the head of the bed, with minimal improvement in symptoms. His physical examination is unremarkable, and he is hemodynamically stable. The patient has no alarm symptoms like dysphagia, odynophagia, weight loss, or gastrointestinal bleeding. Which diagnostic test is used to confirm the diagnosis of GERD and assess the degree of esophageal mucosal injury?