Acquired lumbar spinal stenosis in the elderly is predominantly caused by degenerative changes. The most significant contributors are hypertrophy (thickening) of the ligamentum flavum and degenerative changes in the facet joints (facet arthropathy), which lead to osteophyte formation and capsular hypertrophy. These changes collectively narrow the spinal canal and/or neural foramina. While disc degeneration and bulging (option B) can contribute, they are usually part of the broader degenerative process and less specific as the 'primary' contributor compared to the combined effect of ligamentum flavum and facet changes. Hypertrophy of the anterior longitudinal ligament (option A) is not a primary cause of spinal canal narrowing. Spondylolisthesis due to a pars interarticularis defect (option D) is typically isthmic spondylolisthesis, which is a specific type of instability, and while it can cause stenosis, it's not the primary mechanism for *acquired* degenerative stenosis in the elderly, which is more often due to the aforementioned soft tissue and bony overgrowth.
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