**Core Concept**
Transrectal ultrasonography (TRUS) is a diagnostic imaging technique that utilizes high-frequency sound waves to produce detailed images of the prostate gland and surrounding tissues. This modality is particularly useful for evaluating the prostate gland's size, shape, and texture, as well as detecting abnormalities such as tumors or nodules.

**Why the Correct Answer is Right**
TRUS is widely recognized as the gold standard for prostate cancer staging and diagnosis. The technique allows for the evaluation of the prostate gland's zonal anatomy, which is essential for identifying and characterizing lesions. During TRUS, a transducer is inserted into the rectum, producing high-resolution images of the prostate gland's posterior and lateral aspects. This enables the detection of cancerous lesions, which can be further confirmed by biopsy. The proximity of the transducer to the prostate gland also allows for the accurate measurement of glandular volume and the assessment of glandular homogeneity.

**Why Each Wrong Option is Incorrect**
**Option A:** While TRUS can be used to evaluate the prostate gland's size and texture, it is not primarily used for assessing prostate gland volume.
**Option B:** Although TRUS can be used to guide prostate biopsies, it is not the most important use of this modality.
**Option C:** TRUS is not typically used for imaging the urinary bladder.

**Clinical Pearl / High-Yield Fact**
A key advantage of TRUS is its ability to provide real-time imaging of the prostate gland, which allows for accurate targeting of lesions during biopsy procedures.

**Correct Answer: D. Prostate cancer staging and diagnosis.**