**Core Concept**
The investigation of gross haematuria in an elderly male requires a comprehensive approach to identify the underlying cause, which can range from benign conditions to malignant tumors. The goal is to identify the source of bleeding, assess for any potential obstruction, and detect any signs of malignancy.

**Why the Correct Answer is Right**
Urine tumor markers, such as NMP22 or BTA, are not recommended as a primary investigation for gross haematuria. This is because these markers lack specificity and sensitivity for detecting bladder or upper urinary tract cancers. They can also be elevated in non-malignant conditions, leading to false positives. In contrast, a thorough physical examination, detailed medical history, and imaging studies are essential in evaluating the cause of gross haematuria.

**Why Each Wrong Option is Incorrect**
**Option A:** Cystoscopy is a crucial investigation for evaluating the bladder and urethra for any abnormalities, including tumors or ulcers, which can cause gross haematuria. It allows for direct visualization of the bladder and urethra, making it an essential tool in the investigation of haematuria.

**Option B:** Urine microscopy for malignant cells is a useful investigation to detect any malignant cells in the urine, which can indicate the presence of a bladder or upper urinary tract cancer. It is particularly useful in conjunction with other investigations, such as cystoscopy and imaging studies.

**Option D:** Intravenous Pyelogram (IVP) is a useful imaging study to evaluate the upper urinary tract for any obstruction or abnormalities, which can cause gross haematuria. It can help identify any issues with the kidneys, ureters, or bladder.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with gross haematuria, it is essential to consider the patient's age, medical history, and symptoms to guide the investigation. In elderly males, the risk of bladder and upper urinary tract cancers increases, making a comprehensive investigation essential to identify any underlying malignancy.

**✓ Correct Answer: C. Urine tumor markers**