## **Core Concept**
The posterior urethra is a part of the male urethra that passes through the prostate gland and is surrounded by the urethral sphincter. Visualizing this area is crucial for diagnosing conditions such as benign prostatic hyperplasia (BPH), prostate cancer, and urethral strictures. Various imaging modalities can be used to visualize the urethra, but the best one for the posterior urethra needs to be identified.

## **Why the Correct Answer is Right**
The correct answer, **Cystoscopy**, is the best investigation to visualize the posterior urethra. Cystoscopy involves passing a flexible or rigid scope through the urethra and into the bladder, allowing direct visualization of the urethral lumen, including the posterior urethra. This procedure is highly effective for diagnosing abnormalities such as tumors, strictures, and prostatic enlargement. Cystoscopy can also be used for therapeutic interventions like dilation of strictures or removal of tumors.

## **Why Each Wrong Option is Incorrect**
- **Option A: Ultrasound** - While ultrasound can provide information about the prostate gland and bladder, it is not the best method for directly visualizing the interior of the urethra. Transrectal ultrasound (TRUS) can image the prostate but does not directly visualize the urethral lumen.
- **Option B: MRI** - Magnetic Resonance Imaging (MRI) can provide detailed images of the prostate and surrounding structures but is not typically used for direct visualization of the urethral lumen. MRI can be useful for staging prostate cancer but is not the best choice for detailed urethral inspection.
- **Option D: CT Scan** - Computed Tomography (CT) scans can image the prostate and bladder but are not suitable for detailed visualization of the urethral lumen. CT scans are more commonly used for assessing the extent of disease or evaluating the urinary tract for stones or tumors.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that cystoscopy is not only diagnostic but also therapeutic. For example, during cystoscopy, procedures like urethral dilation or biopsy of urethral lesions can be performed. It's essential to remember that while cystoscopy is excellent for the urethra, the choice of imaging modality depends on the clinical question being asked.

## **Correct Answer: C. Cystoscopy**