**Core Concept**
Isotope renogram, also known as renal scintigraphy, is a nuclear medicine imaging technique used to assess the function and structure of the kidneys. It involves the injection of a radioactive tracer that is excreted by the kidneys, allowing for the evaluation of renal blood flow, glomerular filtration rate, and urine flow.

**Why the Correct Answer is Right**
Isotope renogram is particularly useful in the investigation of hydronephrosis, as it can provide functional information about the affected kidney. The test can help identify the presence of obstruction, assess the severity of the hydronephrosis, and evaluate the function of the affected kidney. This is achieved by analyzing the uptake and clearance of the radioactive tracer by the kidney, which provides information about the kidney's ability to filter and excrete waste. Additionally, isotope renogram can help differentiate between obstructive and non-obstructive hydronephrosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Isotope renogram is not primarily used to assess the size and shape of the kidneys, which is typically done with ultrasound or CT scans.

**Option B:** While isotope renogram can provide some information about the ureters, it is not the most accurate method for evaluating ureteral obstruction or ureteropelvic junction obstruction.

**Option C:** Isotope renogram is not typically used to assess the function of the bladder or the urethra, which is usually done with other imaging modalities such as uroflowmetry or cystoscopy.

**Clinical Pearl / High-Yield Fact**
Isotope renogram is a useful tool for evaluating renal function and structure, particularly in patients with suspected renal obstruction or hydronephrosis. It can provide valuable information that can guide further management and treatment decisions.

**Correct Answer:** C.