**Core Concept**
Painless gross hematuria in a 60-year-old smoker warrants an investigation to rule out urological malignancies, such as bladder or renal cell carcinoma. The investigation of choice should aim to detect malignant cells or other abnormalities in the urine.

**Why the Correct Answer is Right**
Urine for malignant cytology is the investigation of choice because it directly addresses the concern for urological malignancy. Cytological examination of the urine can detect cancer cells, which are often shed into the urine. This is particularly relevant in patients with risk factors such as smoking, age, and gross hematuria. The cytological examination can be performed using a Pap smear-like technique, where the urine is concentrated and examined under a microscope for abnormal cells.

**Why Each Wrong Option is Incorrect**
**Option A:** Urine routine and microscopy may show hematuria but will not directly address the concern for malignancy. While it can provide information on the presence of blood, protein, or other abnormalities, it lacks the specificity to diagnose cancer.
**Option B:** Plain x-ray KUB (Kidneys, Ureters, and Bladder) is not the best initial investigation for painless gross hematuria. It can show calcifications or stones but will not directly address the concern for malignancy.
**Option C:** CT Urogram (USB KUB is an older term) is a more advanced imaging modality that can provide detailed information about the urinary tract. However, it is not typically the first-line investigation for painless gross hematuria and is often reserved for follow-up or in cases where malignancy is suspected based on other investigations.
**Option D (Note: This option is not available, so I've skipped it)**

**Clinical Pearl / High-Yield Fact**
In patients with painless gross hematuria, it's essential to consider urological malignancy, especially in those with risk factors such as smoking. A thorough history, physical examination, and appropriate investigations, including urine cytology, are crucial in making a timely diagnosis and initiating appropriate management.

**✓ Correct Answer: D. Urine for malignant cytology**