Diverticulitis: Given the patient's age, diverticulitis is a common condition that can cause right lower quadrant pain, although it more commonly affects the left lower quadrant. The elderly are at higher risk for complications.

Appendicitis: Although less common in the elderly, appendicitis can occur at any age and may present with atypical symptoms, making diagnosis more challenging.
Cholecystitis: Inflammation of the gallbladder can cause right lower quadrant pain and is more common in women, especially after the age of 40.
Constipation: Elderly individuals are prone to constipation, which can cause abdominal pain, including in the right lower quadrant.
Urinary Tract Infection (UTI): UTIs are common in the elderly and can cause lower abdominal pain, including in the right lower quadrant.

Ectopic Pregnancy: Although highly unlikely in an 89-year-old female, it's theoretically possible in cases of late menopause or with the use of fertility treatments, and would be catastrophic if missed.
Ovarian Torsion: Can occur at any age and requires prompt surgical intervention to preserve the ovary.
Intestinal Obstruction: Can cause severe abdominal pain and is a surgical emergency.
Mesenteric Ischemia: A condition where blood flow to the intestines is blocked, which is more common in the elderly and requires immediate medical attention.

Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis can cause chronic abdominal pain but are less common in the elderly population.
Right Lower Quadrant Tumor: Could be a primary or metastatic tumor causing pain in the right lower quadrant, which would be rare but important to diagnose.
Hernias: Although not typically rare, hernias causing right lower quadrant pain might be less common than other causes but should still be considered, especially if the patient has a history of hernia or surgical interventions.