Chronic Idiopathic Constipation: This condition is characterized by persistent difficulty with defecation, which can lead to severe abdominal distention. The lack of improvement with medications and normal lab results supports this diagnosis, as it often doesn't have an identifiable cause and may not respond to standard treatments.

Irritable Bowel Syndrome (IBS) with Constipation: IBS can cause significant abdominal distention and constipation. Although medications are often used to manage symptoms, some patients may not respond, making this a plausible diagnosis.
Colonic Inertia or Slow Transit Constipation: This condition involves a slowing of the movement of food through the colon, leading to severe constipation and distention. Normal labs would not rule out this condition, which may require specific tests for diagnosis.
Pelvic Floor Dysfunction: Issues such as dyssynergic defecation, where the muscles used for defecation do not coordinate properly, can cause severe constipation and distention. This condition might not be immediately suspected but is a common cause of refractory constipation.

Colorectal Cancer: Although less likely in a 44-year-old without a family history or other risk factors, colorectal cancer can cause constipation and abdominal distention. It's crucial to rule out this diagnosis due to its potential severity and the importance of early detection.
Intestinal Obstruction: Partial or intermittent obstruction can cause severe constipation and distention. This is a medical emergency if not promptly diagnosed and treated.
Neurological Disorders (e.g., Parkinson's Disease, Multiple Sclerosis): Certain neurological conditions can affect bowel motility, leading to severe constipation. These conditions have significant implications for patient management and quality of life.

Hirschsprung's Disease: Typically diagnosed in infancy, late presentations can occur, especially in adults with a long history of untreated constipation. This condition involves an absence of nerve cells in parts of the colon, leading to severe constipation.
Chagas Disease: Caused by a parasite, this condition can lead to megacolon and severe constipation in its chronic phase. It's rare but should be considered in individuals from or who have traveled to endemic areas.
Amyloidosis: A condition where abnormal proteins (amyloid) accumulate in organs, potentially affecting the gastrointestinal system and causing constipation and distention. It's rare and would require specific diagnostic tests.