What is the most likely cause of the 78-year-old man's bilateral buttock pain that worsens with standing?

The most likely cause of the patient's bilateral buttock pain that worsens with standing is lumbar spinal stenosis. This condition is characterized by the narrowing of the spinal canal, which can compress the spinal cord or nerve roots. As a person stands or walks, the increased pressure in the lumbar region can exacerbate symptoms, potentially leading to pain that radiates into the buttocks and legs. This pain commonly improves with bending forward or sitting down, as these positions relieve the pressure on the spinal structures.

In elderly patients, such symptoms are often accompanied by claudication-like features, where pain and discomfort occur with certain activities and improve with rest, similar to what is seen in peripheral arterial disease, but with distinct differences in pain localization and relief positions. The bilateral nature of the buttock pain suggests a central or bilateral issue in the spine rather than unilateral radicular pain that would typically be associated with lumbar disc herniation. Additionally, osteoarthritis of the hip joint usually presents with pain localized to the hip and groin regions rather than prominently in the buttocks, particularly not with the standing position as a primary exacerbating factor.

Therefore, the mechanism and symptoms fit the profile of lumbar spinal stenosis, making it the most likely diagnosis in this scenario