Diverticulitis: This is often the most common cause of left lower quadrant abdominal pain, especially in older adults. It is characterized by inflammation of the diverticula in the colon, leading to pain, tenderness, and possible changes in bowel habits.

Constipation: Can cause significant discomfort and pain in the left lower quadrant due to the accumulation of feces in the sigmoid colon.
Irritable Bowel Syndrome (IBS): A common condition that can cause abdominal pain, bloating, and changes in bowel habits, with pain often localized to the left lower quadrant.
Left Ovarian Cyst or Torsion: In females, ovarian cysts or torsion can cause severe pain that may be localized to the left lower quadrant.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis can cause chronic inflammation and pain in the abdomen, including the left lower quadrant.

Ectopic Pregnancy: Although less common, an ectopic pregnancy can present with abdominal pain, including in the left lower quadrant, and is a medical emergency.
Appendicitis (if appendix is located in the left lower quadrant): Although appendicitis typically presents with right lower quadrant pain, anatomic variations can lead to pain in the left lower quadrant.
Intestinal Obstruction: Can cause severe abdominal pain, vomiting, and constipation, and requires immediate medical attention.
Ruptured Abdominal Aortic Aneurysm: Presents with severe abdominal pain and is a life-threatening condition.

Colonic Cancer: Although not typically presenting with acute pain, colonic cancer can cause chronic abdominal pain and changes in bowel habits.
Endometriosis: Can cause chronic pelvic pain, including in the left lower quadrant, especially during menstruation.
Infectious Colitis: Infections like Salmonella or Shigella can cause abdominal pain, diarrhea, and fever.
Lymphoma: Can cause abdominal pain, weight loss, and other systemic symptoms, though it is less common as a cause of left lower quadrant pain.