**Core Concept**
Painless gross hematuria in a chronic smoker is a red flag for urothelial carcinoma, particularly in the bladder. The investigation must detect malignant cells early to guide timely intervention, as early diagnosis significantly impacts prognosis.

**Why the Correct Answer is Right**
Urine microscopy for malignant cytology cells (e.g., cytology of urine sediment) is the most sensitive and specific test to identify bladder cancer cells in the setting of painless hematuria. It directly detects abnormal epithelial cells, especially in high-risk patients like chronic smokers. This test is more accurate than imaging for early-stage tumors and is a cornerstone in diagnosing urothelial malignancy when hematuria is unexplained.

**Why Each Wrong Option is Incorrect**
Option A: USG (ultrasound of the kidneys and bladder) may detect mass or obstruction but lacks sensitivity for early malignancy and cannot identify malignant cells.
Option B: X-ray KUB is outdated and has poor sensitivity for detecting bladder cancer or early lesions; it is not specific for hematuria of malignant origin.
Option C: Urine routine is non-specific and may only show RBCs without differentiating between benign and malignant causes.

**Clinical Pearl / High-Yield Fact**
In patients over 50 with chronic smoking history and painless gross hematuria, urine cytology is the **first-line investigation** for detecting bladder cancer. A negative cytology does not rule out malignancy, but a positive result is highly specific and warrants further imaging or cystoscopy.

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