An 88 year old man presents with a sensation of burning in his mouth. He has a history of chronic obstructive pulmonary disease, and has had several courses of antibiotics recently. He has lost weight and eats poorly. His body mass index is 18. What is the most likely cause of his symptoms?
1) Oral thrush
2) Gastroesophageal reflux disease (GERD)
3) Oral cancer
4) Dry mouth