What is the most likely underlying cause of urinary symptoms in an elderly man with diabetes and bladder diverticulum?

In the context of an elderly man with diabetes and bladder diverticulum, the underlying cause of urinary symptoms is most likely benign prostatic hyperplasia (BPH). As men age, it is common for the prostate gland to undergo hyperplastic changes, which can lead to a variety of obstructive urinary symptoms such as increased frequency, urgency, and difficulty initiating urination.

In patients with bladder diverticula, which are pouches that form in the bladder wall, any obstructive symptoms from BPH can exacerbate urinary issues. The presence of diabetes can further complicate urinary function, but BPH typically stands out as a primary factor influencing urinary symptoms in elderly men. The mechanical obstruction caused by an enlarged prostate can lead to urinary retention and increased risk of urinary tract infections, particularly when combined with the existing diverticulum, which can serve as a reservoir for bacteria.

Hypertension, nephrolithiasis, and type 2 diabetes mellitus can contribute to urinary symptoms, but they do not directly explain the immediate obstructive phenomena typically associated with BPH in this demographic. Thus, benign prostatic hyperplasia is the most relevant cause in the scenario presented.