Irritable Bowel Syndrome (IBS): This is often the most common cause of abdominal pain associated with defecation, characterized by a change in bowel habits, bloating, and abdominal discomfort without an identifiable cause.

Hemorrhoids: Pain during defecation can be due to hemorrhoids, especially if they are thrombosed. Symptoms include pain, itching, and bleeding.
Anal Fissure: A tear in the lining of the anus can cause severe pain during and after defecation, often accompanied by bleeding.
Diverticulitis: Inflammation of the diverticula in the colon can cause abdominal pain, changes in bowel habits, and possibly bleeding.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can cause abdominal pain, diarrhea, weight loss, and bleeding.

Colorectal Cancer: Although less common, colorectal cancer can present with abdominal pain, changes in bowel habits, and bleeding. It's crucial to rule out this diagnosis, especially in older adults or those with a family history.
Intestinal Obstruction: A blockage in the intestine can cause severe abdominal pain, vomiting, and constipation. This is a medical emergency.
Appendicitis: While appendicitis typically presents with right lower quadrant pain, it can sometimes cause pain that worsens with defecation. It's a surgical emergency.
Infectious Colitis: Infections like Clostridioides difficile (C. diff) can cause severe diarrhea, abdominal pain, and potentially life-threatening complications if not treated promptly.

Intussusception: A condition where a part of the intestine slides into an adjacent part, causing bowel obstruction. It's rare in adults but can cause abdominal pain, vomiting, and bloody stools.
Colonic Volvulus: A twisting of the colon that can cause obstruction, severe abdominal pain, and vomiting. It requires emergency treatment.
Proctalgia Fugax: A condition characterized by sudden, severe anal pain that lasts for seconds or minutes, often at night. It's benign but can be very painful.