Final answer:
The most likely diagnosis for a 75-year-old man with long-standing constipation and a new onset of abdominal pain and distention is sigmoid volvulus, a condition in which the sigmoid colon becomes twisted and causes a blockage.
Step-by-step explanation:
A 75-year-old man with a long-standing history of constipation presenting with abdominal pain and distention is most likely experiencing a sigmoid volvulus. This condition is characterized by a twist in the sigmoid colon, which is the part of the large intestine that is shaped like the letter 'S' and connects to the rectum. The twist can cause a blockage, leading to symptoms such as pain and distention. Sigmoid volvulus is more common in older adults and can be associated with chronic constipation.
Other conditions, such as cecal volvulus, intussusception, and irritable bowel syndrome, may also cause similar symptoms but are less likely given the patient's age and symptom profile. The most likely diagnosis for a 75-year-old man with a long-standing history of constipation, abdominal pain, and distention is d) sigmoid volvulus. Sigmoid volvulus occurs when the sigmoid colon twists upon itself, causing a blockage and trapping stool. This can result in symptoms such as severe abdominal pain, constipation, and distention.