Final answer:
The elderly male patient's symptoms of abdominal pain, constipation, and distension with an X-ray showing air-fluid levels and intestinal distention are indicative of diverticulitis, an infection and inflammation of the diverticula in the large intestine.
Step-by-step explanation:
The most common cause of the patient's problem, described as abdominal pain, progressive for two months, with associated constipation and abdominal distension with air-fluid levels and distention of intestines with haustral markings, is likely diverticulitis. Diverticulitis is a condition where the tiny pouches, called diverticula, which are present in the wall of the large intestine, become infected and inflamed, leading to a variety of symptoms including changes in bowel habits and abdominal pain.
Diverticulosis, the presence of these pouches without inflammation, is linked to both genetic and environmental factors, such as obesity, and can evolve into diverticulitis in some individuals. The presence of diverticula is more common as people age, and the subsequent infection and inflammation in diverticulitis are typically caused by bacteria. Considering the patient's age and the description of his symptoms, diverticulitis is a probable diagnosis.