Diverticulitis: This is often the most common cause of left iliac fossa pain, especially in older adults. It occurs when diverticula in the wall of the colon become inflamed, leading to pain, tenderness, and potentially more severe complications if not addressed.

Irritable Bowel Syndrome (IBS): Characterized by chronic abdominal pain, bloating, and alteration of bowel habits, IBS can cause left iliac fossa pain, though it's typically more diffuse and accompanied by other gastrointestinal symptoms.
Constipation: Severe constipation can cause significant abdominal pain, including in the left iliac fossa, due to the accumulation of feces in the sigmoid colon.
Gastroenteritis: Inflammation of the gastrointestinal tract, usually due to infection, can cause abdominal pain, including in the left iliac fossa, along with diarrhea, vomiting, and fever.

Ectopic Pregnancy: Although more commonly associated with right iliac fossa pain due to the typical location of the appendix, an ectopic pregnancy can present with pain on either side, depending on the implantation site of the embryo. It's a medical emergency.
Intestinal Obstruction: Obstruction of the intestine can cause severe abdominal pain, vomiting, and constipation. The pain can be localized to the left iliac fossa if the obstruction is in the sigmoid or descending colon.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can cause chronic inflammation and pain in the abdomen, including the left iliac fossa, along with diarrhea, weight loss, and other symptoms.
Ovarian Cyst or Torsion: In females, ovarian cysts can cause pain if they rupture or torsion occurs, leading to an acute abdomen. The pain is typically severe and may be localized to one side.

Splenic Infarction or Rupture: Although the spleen is located in the left upper quadrant, splenic infarction or rupture can refer pain to the left iliac fossa due to shared nerve roots.
Left Kidney Stone or Pyelonephritis: While kidney stones and pyelonephritis typically cause flank pain, they can occasionally radiate to the lower abdomen, including the left iliac fossa.
Colonic Cancer: Though less common as an initial presentation, colonic cancer can cause localized pain, especially if the tumor is large or has invaded surrounding structures. It's more likely to be considered in older patients or those with risk factors.