Diverticulitis: This condition is a common cause of left lower abdominal pain, especially in older adults. It occurs when diverticula in the colon become inflamed, leading to pain, tenderness, and potentially serious complications if not treated.

Constipation: Constipation can cause significant abdominal pain and discomfort, often localized to the lower abdomen. It's a common issue that can be exacerbated by diet, lifestyle, and certain medications.
Irritable Bowel Syndrome (IBS): IBS can cause a variety of symptoms including abdominal pain, changes in bowel movements, and bloating. The pain is often crampy and can be localized to the left lower abdomen.
Ovarian Cyst: In women, an ovarian cyst can cause left lower abdominal pain if the cyst is located on the left ovary. These cysts are usually benign but can cause significant pain if they rupture or twist.
Kidney Stone: A kidney stone passing down the ureter can cause severe, colicky pain that may be felt in the left lower abdomen if the stone is in the left ureter.

Ectopic Pregnancy: Although less common, an ectopic pregnancy is a life-threatening condition that must be considered in women of childbearing age presenting with abdominal pain. The pain can be localized to one side of the abdomen.
Appendicitis: While appendicitis typically causes right lower abdominal pain, in some cases (especially if the appendix is retrocecal), the pain can be more diffuse or even localized to the left lower abdomen.
Intestinal Obstruction: An obstruction in the intestines can cause severe abdominal pain, vomiting, and constipation. It's a medical emergency that requires prompt diagnosis and treatment.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can cause chronic abdominal pain, diarrhea, and weight loss. They are important to diagnose early to prevent complications.

Endometriosis: While not exclusively causing left lower abdominal pain, endometriosis can lead to chronic pelvic pain, including pain localized to one side of the abdomen, especially during menstruation.
Lymphoma: Abdominal lymphoma can cause pain, weight loss, and other systemic symptoms. It's a rare cause of left lower abdominal pain but should be considered in the differential diagnosis, especially in patients with risk factors.
Intra-abdominal Testicular Torsion: In cases of undescended testes, torsion can occur within the abdomen and cause severe, acute abdominal pain that may be localized to one side.
Splenic Infarction or Rupture: Conditions affecting the spleen, such as infarction or rupture, can cause left upper quadrant pain but may also refer pain to the left lower abdomen in some cases.