Q. A 65-year-old man comes to the clinic because of a longstanding history of difficulty swallowing and his family is always commenting about his bad-smelling breath. Particularly frightening is the fact that he can wake up next to what appears to be regurgitated food that he swears is even from several days ago. He is a heavy smoker, but he does not drink alcohol. His BP is 140/85 mmHg, HR is 90/min Temperature is 37.2 °C, and RR is 18/min. Cardiopulmonary and abdominal examinations are normal. A barium swallow is ordered, it reveals an out-pouching sac on the posterior pharyngeal wall (see image). What is the most likely cause of this condition?