**Core Concept**
Painless gross hematuria refers to the sudden onset of visible blood in the urine without any associated pain or discomfort. This symptom can be caused by a variety of conditions affecting the genitourinary system, including benign or malignant tumors, urinary tract infections, or vascular abnormalities.

**Why the Correct Answer is Right**
The most common cause of painless gross hematuria is a urothelial carcinoma, also known as a transitional cell carcinoma (TCC). TCC arises from the lining of the urinary system, including the renal pelvis, ureters, bladder, or urethra. The presence of blood in the urine is often the first sign of this malignancy, and it typically occurs in older adults. The exact mechanism of TCC involves genetic mutations in the TP53 tumor suppressor gene, leading to uncontrolled cell growth and tumor formation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because glomerulonephritis, a disease of the glomeruli, typically presents with microscopic hematuria and/or proteinuria, not gross hematuria.
**Option B:** This option is incorrect because a kidney stone or nephrolithiasis usually causes painful hematuria due to irritation of the ureteral mucosa.
**Option C:** This option is incorrect because interstitial cystitis, also known as bladder pain syndrome, is characterized by recurrent painful bladder symptoms, not painless gross hematuria.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that painless gross hematuria in an older adult should raise suspicion for a urothelial carcinoma, and prompt further investigation, such as cystoscopy and urine cytology, is essential to confirm the diagnosis.

**Correct Answer:** D.