Adhesional bowel obstruction: This is the most common cause of small bowel obstruction in adults, often resulting from post-surgical adhesions. The likelihood is high due to the frequency of abdominal surgeries and the subsequent formation of adhesions.

Hernias (inguinal, femoral, umbilical, or internal): These can cause bowel obstruction, especially if the bowel becomes incarcerated or strangulated within the hernia. The risk varies with the type and size of the hernia.
Volvulus: A condition where the bowel twists around its mesentery, cutting off its blood supply. This is more common in certain populations, such as the elderly or those with anatomical abnormalities.
Intussusception: Although more common in children, intussusception can occur in adults, often due to a lead point like a tumor. It is less common than other causes but should be considered, especially if there's a mass effect or malignancy suspected.
Crohn's disease or other inflammatory bowel diseases: These conditions can cause strictures that lead to bowel obstruction. The likelihood depends on the severity and extent of the disease.

Mesenteric ischemia: While not a direct cause of mechanical bowel obstruction, mesenteric ischemia can present with similar symptoms and is a medical emergency requiring prompt intervention to prevent bowel necrosis.
Strangulated bowel: Any condition leading to bowel ischemia due to compromised blood supply is critical and requires immediate surgical intervention to prevent necrosis and perforation.
Malignancy: Although less common, bowel obstruction can be the presenting symptom of a malignancy, such as colorectal or ovarian cancer. Missing this diagnosis could significantly impact prognosis and treatment.

Gallstone ileus: A rare condition where a gallstone enters the bowel through a fistula and obstructs the intestine. It's more common in elderly women with a history of gallstones.
Foreign body obstruction: This can occur, especially in children or in cases of ingestion of large or inappropriate items.
Retroperitoneal fibrosis: A rare condition that can cause ureteral and bowel obstruction due to fibrotic tissue encasement.
Internal hernias through congenital or acquired defects: These are less common and can be challenging to diagnose without a high index of suspicion and appropriate imaging.