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 the importance of not missing this diagnosis due to its high mortality rate if not promptly treated.
Appendiceal Rupture: If appendicitis is not treated promptly, the appendix can rupture, leading to peritonitis, which is a life-threatening condition.
Ovarian Torsion: Failing to diagnose ovarian torsion can lead to the loss of the ovary due to lack of blood supply, which can have significant long-term fertility implications.
Intestinal Obstruction: This can cause severe abdominal pain, nausea, vomiting, and constipation. If not treated, it can lead to bowel ischemia and perforation.

Meckel's Diverticulitis: A rare condition where a small bulge in the small intestine (Meckel's diverticulum) becomes inflamed, causing pain, often in the right lower abdomen.
Intussusception: A condition where a part of the intestine slides into an adjacent part, causing bowel obstruction and potentially severe pain.
Right Lower Lobe Pneumonia: Sometimes, pneumonia affecting the right lower lobe of the lung can cause referred pain to the right lower abdomen.
Inflammatory Bowel Disease (Crohn's Disease or Ulcerative Colitis): While not typically presenting with acute pain, these conditions can cause chronic right lower quadrant pain among other symptoms.