**Core Concept**
Pelvi-ureteric junction obstruction is a condition where there is a blockage at the junction between the renal pelvis and the ureter, leading to **hydronephrosis**. This condition can cause renal impairment due to the obstruction of urine flow. The diagnosis of this condition requires imaging studies to visualize the urinary tract.

**Why the Correct Answer is Right**
The best diagnostic tool for pelvi-ureteric junction obstruction is not specified in the options, but typically, **Diuretic Renography** is considered the gold standard for diagnosing this condition. It involves the use of a radioactive tracer and a diuretic to assess the function and drainage of the kidneys.

**Why Each Wrong Option is Incorrect**
**Option A:** Without the specific option text, it's challenging to provide a detailed explanation, but generally, options that do not involve imaging or functional assessment of the kidneys are less likely to be correct.
**Option B:** Similarly, without the text, it's hard to comment, but options that suggest invasive procedures might not be the first line for diagnosis.
**Option C:** Again, lacking the text, but if it suggests a basic laboratory test, it might not provide enough information for a definitive diagnosis.
**Option D:** If this option suggests an alternative imaging technique, it might not be as specific or sensitive as the preferred method for diagnosing pelvi-ureteric junction obstruction.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **Diuretic Renography** can help differentiate between obstructive and non-obstructive causes of hydronephrosis, making it a crucial diagnostic tool in such cases.

**Correct Answer:** Unfortunately, without the provided answer choices, a specific correct answer cannot be given.