Appendicitis: This is the most common cause of right lower abdominal pain, especially if the pain is severe, constant, and accompanied by fever, nausea, and vomiting. The pain often starts near the navel and then moves to the right lower abdomen.

Ovarian Cyst or Torsion: In females, ovarian cysts or torsion can cause severe right lower abdominal pain, often accompanied by nausea and vomiting.
Ectopic Pregnancy: Another possible cause in females, especially if there's a history of missed periods or positive pregnancy test, presenting with severe abdominal pain and potentially vaginal bleeding.
Diverticulitis: Inflammation of the diverticula in the colon can cause pain in the lower abdomen, which can be on the right side if the diverticula are located there.
Kidney Stone: A stone passing down the ureter can cause severe, colicky pain that may be felt in the right lower abdomen if the stone is in the right ureter.
Constipation: Severe constipation can cause abdominal pain, including in the right lower quadrant, especially if there is a significant amount of stool in the colon.

Ruptured Ectopic Pregnancy: Although less common, a ruptured ectopic pregnancy is a medical emergency that requires immediate attention. Symptoms include severe abdominal pain, vaginal bleeding, and signs of shock.
Perforated Visculitis or Diverticulitis: Perforation can lead to peritonitis, a life-threatening condition requiring immediate surgical intervention.
Intestinal Obstruction: Obstruction of the intestines can cause severe abdominal pain, vomiting, and constipation. It's a medical emergency if the obstruction is complete or if there are signs of bowel ischemia.
Pneumonia (Basilar): Sometimes, basilar pneumonia can refer pain to the abdomen, mimicking an acute abdominal condition.

Inflammatory Bowel Disease (Crohn's Disease or Ulcerative Colitis): While not the first consideration, these conditions can cause chronic abdominal pain, including in the right lower quadrant, along with changes in bowel habits and weight loss.
Endometriosis: Can cause chronic pelvic pain, including right lower abdominal pain, especially during menstruation.
Lymphoma: Abdominal lymphoma can cause pain, although it's less common and usually accompanied by other systemic symptoms like weight loss and fever.
Right Lower Lobe Pneumonia: Similar to basilar pneumonia, right lower lobe pneumonia can cause referred pain to the right lower abdomen.