Constipation with possible fecal impaction: Given the patient's symptoms of nausea, abdominal pain, and inability to pass a bowel movement for 3 days, constipation with possible fecal impaction is a common and likely diagnosis, especially in elderly patients who may have decreased mobility, poor diet, or medication side effects contributing to constipation.

Diverticulitis: Inflammation of the diverticula in the colon can cause abdominal pain, nausea, and changes in bowel habits, including constipation.
Gastroenteritis: Viral or bacterial infection of the stomach and intestines can lead to nausea, abdominal pain, and constipation, although diarrhea is more common.
Intestinal obstruction: Partial or complete blockage of the intestine can cause severe abdominal pain, nausea, vomiting, and constipation.
Inflammatory bowel disease (IBD): Conditions like Crohn's disease or ulcerative colitis can cause chronic inflammation of the gastrointestinal tract, leading to abdominal pain, nausea, and changes in bowel habits.

Appendicitis: Although less common in elderly patients, appendicitis can present with atypical symptoms, including constipation, and is a surgical emergency if not promptly treated.
Mesenteric ischemia: Reduced blood flow to the intestines can cause severe abdominal pain, nausea, and constipation, and is a life-threatening condition if not promptly diagnosed and treated.
Ovarian torsion: Twisting of the ovary can cause severe abdominal pain, nausea, and vomiting, and is a surgical emergency.

Colonic volvulus: Twisting of the colon can cause abdominal pain, nausea, and constipation, and is a rare but serious condition that requires prompt treatment.
Intussusception: Telescoping of one segment of intestine into another can cause abdominal pain, nausea, and constipation, and is a rare condition in adults.
Neurogenic bowel dysfunction: Conditions like Parkinson's disease or spinal cord injury can cause neurogenic bowel dysfunction, leading to constipation and other bowel symptoms.