Q. A 66-year-old man comes to the ER because of a 2-month history of bloody cough with constipation and generalized weakness. He says that his weakness improves after walking 5 minutes around the house. His wife says he walks as if "he is holding back from going to the bathroom." He has smoked 1.5 packs of cigarettes daily for the past 50 years. Deep tendon reflexes are decreased on exam. His temperature is 37.4 °C, BP is 140/80, and breathing rate is 21/minute. Chest X-ray reveals a mass that is located in the central region of the right lung. What is the most likely cause of constipation and weakness in this patient?