Carcinoma of the terminal ileum or ileocecal junction: This is a common cause of bowel obstruction in the elderly, especially given the location at the terminal ileum. The justification for this being the most likely diagnosis is the age of the patient and the specific location of the obstruction, which is a common site for tumors to develop.

Adhesions from previous abdominal surgery: These are a common cause of bowel obstruction, especially in patients with a history of abdominal operations. The elderly patient may have had previous surgeries, making adhesions a plausible cause.
Inflammatory bowel disease (Crohn's disease or ulcerative colitis): Although these conditions can present at any age, they are less common in the elderly. However, they can cause strictures leading to obstruction, especially if the disease is long-standing.
Hernias (internal or external): Hernias can cause bowel obstruction, especially if they involve the small intestine. The terminal ileum can be involved in internal hernias or volvulus.

Intussusception: Although rare in adults, intussusception can cause bowel obstruction and is more common in the elderly. It requires prompt diagnosis and treatment to avoid complications.
Ischemic bowel disease: This condition can lead to bowel obstruction due to ischemia causing a stricture or due to the resulting inflammation. It's critical to diagnose early to prevent bowel necrosis.
Foreign body or bezoar: Though less common, a foreign body or bezoar can obstruct the bowel, especially in the setting of decreased mobility or altered mental status in the elderly.

Lymphoma: Can cause bowel obstruction by either external compression or by infiltrating the bowel wall, leading to a stricture.
Gastrointestinal stromal tumors (GISTs): These are rare tumors of the GI tract that can cause obstruction, usually by external compression.
Meckel's diverticulum: A congenital anomaly that can cause obstruction, usually by acting as a lead point for intussusception or by causing a volvulus.

Imaging: CT scan of the abdomen with oral and IV contrast is usually the first step to evaluate the cause of obstruction, visualize the bowel, and identify any masses or other abnormalities.
Endoscopy: Colonoscopy or enteroscopy may be useful depending on the suspected cause, especially if a tumor or inflammatory bowel disease is considered.
Surgical consultation: Given the high likelihood of requiring surgical intervention for both diagnosis and treatment, early consultation with a surgeon is advisable.
Laboratory tests: Complete blood count, electrolyte panel, and blood urea nitrogen/creatinine to assess for signs of infection, dehydration, or renal impairment. Tumor markers may be considered based on the clinical suspicion of malignancy.