**Core Concept**
Painless gross hematuria in a 60-year-old smoker warrants a thorough investigation for urological malignancies, particularly transitional cell carcinoma of the bladder. This condition requires a prompt and accurate diagnosis to initiate timely treatment.

**Why the Correct Answer is Right**
Urine microscopy for malignant cytology is the most logical investigation in this scenario because it allows for the direct examination of urinary cells for abnormal morphology, which can indicate cancer. This test is particularly useful in cases of painless gross hematuria, as it can detect malignant cells that may not be visible on routine urine analysis. The presence of a smoking history further increases the likelihood of urothelial cancer, making this test the most appropriate initial step in diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Urine routine may reveal some abnormalities such as blood or protein, but it will not provide a definitive diagnosis of cancer. Routine urine analysis lacks the sensitivity to detect malignant cells.
**Option B:** Plain X-ray KUB (Kidneys, Ureters, and Bladder) is useful for detecting radio-opaque stones or calcifications but is not the best initial investigation for painless gross hematuria.
**Option C:** USG KUB (Ultrasound of the Kidneys, Ureters, and Bladder) is a useful imaging modality for detecting masses or obstruction but may not provide a definitive diagnosis of cancer.
**Option D:** This option is actually the correct answer, so we'll skip it.

**Clinical Pearl / High-Yield Fact**
In patients presenting with painless gross hematuria, a high index of suspicion for urothelial cancer should be maintained, and urine microscopy for malignant cytology should be performed promptly.

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