A sprightly 82 year old woman with a past history of a small hiatus hernia is recently diagnosed with osteoporosis and started on appropriate treatment. Three months later she complains to her family physician during a routine visit of progressive dysphagia to solids, especially to meat. The family physician notes that she has lost 3 kg in weight although she retains a good appetite. She has no other symptoms and clinical examination is otherwise unremarkable. What is the most likely explanation?