**Core Concept**
The question requires the diagnosis of a condition causing recurrent urinary tract infections (UTIs) in a 40-year-old male, with an X-ray KUB showing a radio-opaque shadow. The underlying concept involves identifying a condition that affects the urinary system, leading to recurrent infections and visible abnormalities on imaging.

**Why the Correct Answer is Right**
The presence of a radio-opaque shadow on the X-ray KUB, combined with recurrent UTIs, suggests a condition affecting the urinary system. A possible diagnosis is a kidney stone (nephrolithiasis), which can cause recurrent UTIs due to obstruction of urine flow and bacterial growth. Kidney stones are composed of minerals and salts that form in the kidneys and can be visible on X-ray KUB due to their radio-opacity.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not account for the radio-opaque shadow on X-ray KUB, which is a key diagnostic clue.
* **Option B:** This option is incorrect because it does not address the recurrent nature of the UTIs, which suggests a structural or functional abnormality in the urinary system.
* **Option C:** This option is incorrect because it does not consider the possibility of a kidney stone, which is a common cause of recurrent UTIs and radio-opaque shadows on X-ray KUB.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that kidney stones are a common cause of recurrent UTIs in adults, particularly in males. The presence of a radio-opaque shadow on X-ray KUB should raise suspicion for a kidney stone.

**Correct Answer: C. Kidney stone (nephrolithiasis)**