Benign Prostatic Hyperplasia (BPH): The patient's age and symptoms of urinary frequency, hesitancy, and nocturia are classic for BPH. The presence of trace blood in the urinalysis can also be seen in BPH due to prostatic urethral irritation.

Chronic Prostatitis: This condition can cause similar urinary symptoms and flank pain, especially if the prostatitis is affecting the left side, which could radiate to the flank.
Urinary Tract Infection (UTI): Although UTIs are more common in women, older men can also develop UTIs, which could explain the urinary frequency, hesitancy, and trace blood in the urine.
Kidney Stones: The left flank pain could be indicative of a kidney stone, especially if it's causing obstruction and subsequent infection or irritation leading to the urinary symptoms and hematuria.

Prostate Cancer: While less likely than BPH, prostate cancer must be considered, especially in an older male with urinary symptoms and hematuria. It's crucial to rule out cancer due to its significant implications.
Renal Cell Carcinoma: This is another critical diagnosis not to miss, given the patient's age and the presence of flank pain and hematuria, which are classic symptoms of renal cell carcinoma.
Pyelonephritis or Perinephric Abscess: These conditions can present with flank pain and urinary symptoms. Given the potential for severe consequences if not treated promptly, they should be considered.

Bladder Cancer: Although less common than prostate cancer, bladder cancer could explain the hematuria and some of the urinary symptoms, especially if the cancer is obstructing the flow of urine.
Interstial Cystitis: This condition can cause urinary frequency, urgency, and pain, but it's less likely in men and doesn't typically cause flank pain or hematuria.
Polycystic Kidney Disease (PKD): While PKD can cause flank pain and hematuria, it's a genetic condition that usually presents earlier in life, making it a less likely diagnosis for a 73-year-old with a two-year history of symptoms.