Okay, let's tackle this question. The patient has primary infertility with low semen volume, fructose-negative ejaculate, and azoospermia. The question is asking for the most useful imaging modality to evaluate the cause.

First, I need to recall the possible causes of these findings. Low semen volume and absence of fructose suggest a problem in the seminal vesicles, which produce fructose. Azoospermia means no sperm in the ejaculate, which could be due to obstruction or testicular failure. But since fructose is also low, obstruction is more likely because the seminal vesicles aren't contributing, leading to low volume and no fructose. The seminal vesicles are part of the ejaculatory duct system. So, obstruction here could be from congenital absence, infection, or blockage.

The next step is imaging. The options aren't listed, but common imaging modalities for this area would be ultrasound (transrectal or scrotal), MRI, or CT. Transrectal ultrasound (TRUS) is often used to evaluate the seminal vesicles and ejaculatory ducts. It can detect blockages, cysts, or other abnormalities in those structures. MRI might provide more detailed images but is less commonly used as a first-line. CT could show some structural issues but isn't as detailed for soft tissues. Scrotal ultrasound would check for testicular issues, but since the problem here is in the seminal vesicles, TRUS is more appropriate.

So, the correct answer is likely transrectal ultrasound. The other options would be incorrect because scrotal ultrasound doesn't assess the seminal vesicles directly, MRI is more expensive and not first-line, and CT isn't as good for soft tissue imaging in this context. The clinical pearl here is that fructose in semen is a marker of seminal vesicle function, and its absence suggests obstruction distal to the vas deferens. TRUS is the imaging modality of choice for evaluating the seminal vesicles and ejaculatory ducts.

**Core Concept**
This question tests the evaluation of male infertility due to obstructive azoospermia. Low semen volume and fructose deficiency suggest obstruction distal to the vas deferens (e.g., ejaculatory duct or seminal vesicles), as fructose is produced by seminal vesicles and transported via the ejaculatory duct.

**Why the Correct Answer is Right**
Transrectal ultrasound (TRUS) is the most useful imaging modality to visualize the seminal vesicles, ejaculatory ducts, and prostate. It can detect congenital absence, cysts, or obstruction in these structures, which are common causes of obstructive azoospermia with fructose-negative ejaculate. TRUS provides high-resolution images of the retroperitoneal and pelvic anatomy critical to semen production.

**Why Each Wrong Option is Incorrect**
**Option A:** Scrotal ultrasound evaluates testes and epididymis but not the seminal vesicles or ejaculatory ducts.
**Option C:** CT lacks the resolution to detail seminal vesicle anatomy and exposes patients to unnecessary radiation.
**Option D:** MRI is less accessible and more costly than TRUS, making it a second-line option for complex cases only.

**Clinical Pearl / High-Yield Fact**
Fructose in semen is a key marker of seminal vesicle function. Its absence with azoospermia and low volume strongly suggests

✓ Correct Answer: B. Transrectal ultrasonography