Appendicitis: This is the most common cause of right lower quadrant 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 can cause right lower abdomen pain, especially if the cyst ruptures or undergoes torsion. Symptoms can include sudden, severe pain, and sometimes nausea and vomiting.
Ectopic Pregnancy: Another condition in females, an ectopic pregnancy can cause right lower abdomen pain, especially if the pregnancy ruptures. Symptoms can include severe pain, vaginal bleeding, and sometimes shoulder pain due to blood irritating the diaphragm.
Diverticulitis: Inflammation of a diverticulum in the colon can cause pain in the right lower abdomen, although it more commonly affects the left lower quadrant. Symptoms include pain, changes in bowel habits, and possibly fever.
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. The pain can radiate to the groin and is often accompanied by nausea and vomiting.

Ruptured Ectopic Pregnancy: Although mentioned earlier, it's crucial to emphasize that a ruptured ectopic pregnancy is a medical emergency requiring immediate attention. It can present with sudden, severe abdominal pain, vaginal bleeding, and signs of shock.
Appendiceal Rupture: If appendicitis is not treated promptly, the appendix can rupture, leading to peritonitis, which is inflammation of the lining of the abdominal cavity. This is a life-threatening condition requiring immediate surgical intervention.
Ovarian Torsion: This condition can cut off blood supply to the ovary, leading to tissue death if not promptly treated. It presents with severe, sudden pain and sometimes nausea and vomiting.
Intestinal Obstruction: Although it can cause more diffuse abdominal pain, an obstruction in the small or large intestine can sometimes present with pain localized to the right lower quadrant, especially if the obstruction is in the terminal ileum or cecum.

Meckel's Diverticulitis: A rare condition where a congenital diverticulum of the small intestine becomes inflamed, causing right lower quadrant pain, fever, and sometimes nausea and vomiting.
Intussusception: A condition where a part of the intestine slides into an adjacent part, which can cause intestinal obstruction and ischemia. It's rare in adults but can cause severe abdominal pain, including in the right lower quadrant.
Right Lower Lobe Pneumonia: Sometimes, pneumonia affecting the right lower lobe of the lung can cause referred pain to the right lower abdomen, especially if the inflammation irritates the diaphragm.