Gilbert's Syndrome: A benign condition characterized by a mild increase in unconjugated bilirubin, often exacerbated by stress, fasting, or illness. It's common and can present at any age, including in the elderly, especially if previously undiagnosed due to its mild nature.

Medication-Induced Hyperbilirubinemia: Many drugs can cause an elevation in bilirubin levels, either by increasing bilirubin production or by affecting its metabolism and excretion. Given the age of the patient, polypharmacy is a common issue, increasing the likelihood of drug-induced side effects.
Hemolysis: Mild hemolysis can lead to an increase in bilirubin. Conditions such as autoimmune hemolytic anemia or hereditary spherocytosis could be considered, though they might be less common in this age group without a previous diagnosis.
Chronic Liver Disease: Conditions like chronic hepatitis or early stages of cirrhosis can cause mild elevations in bilirubin. The elderly might have a higher likelihood of liver disease due to cumulative exposure to liver toxins or the presence of other health conditions.

Hemolytic Disease of the Newborn (Unlikely but Consider in Specific Contexts): Though extremely rare in a 78-year-old, if the patient has recently received a blood transfusion, a hemolytic reaction could occur. This is more of a consideration in the context of recent medical interventions rather than a spontaneous condition.
Pancreatic Cancer: Obstructive jaundice due to pancreatic cancer can initially present with mild elevation of bilirubin. Given the age of the patient, pancreatic cancer is a significant concern and should not be missed.
Cholangiocarcinoma: Similar to pancreatic cancer, cholangiocarcinoma can cause obstructive jaundice. Early detection is crucial, and any unexplained elevation in bilirubin in an elderly patient warrants further investigation.

Dubin-Johnson Syndrome: A rare genetic disorder causing chronic jaundice due to an inability to secrete conjugated bilirubin into the bile. It's extremely rare and usually diagnosed earlier in life, but could be considered if other causes are ruled out.
Rotor Syndrome: Similar to Dubin-Johnson Syndrome, Rotor Syndrome is a rare, benign condition that affects the liver's ability to handle bilirubin, leading to mild jaundice. It's another condition typically identified earlier in life but could be a rare cause in an elderly patient without a previous diagnosis.
Zellweger Spectrum Disorders: A group of rare, genetic disorders affecting the liver and other parts of the body. While typically diagnosed in infancy or early childhood, mild forms might present later in life with non-specific symptoms, including elevated bilirubin.