Semen analysis of a young man who presented with primary infertility revealed low volume, fructose negative ejaculate with azoospermia. Which of the following is the most useful imaging modality to evaluate the cause of his infertility?
First lets briefly review the anatomy of male internal genitalia
Sperms are produced in testes and then stored and mature within the epididymis. Vas deferens carries the sperms from epididymis to the urethra where they open by separate openings into the prostatic urethra. Just before opening each vas deferens is joined by ducts of seminal vesicles. Vas deferens and seminal vesicle ducts join to form the ejaculatory duct.
The secretions of seminal vesicles form a large part of seminal fluid and contain fructose and a coagulating enzyme called the vesiculase.

Now coming to the question.

Semen analysis forms an important part of infertility assessment.
Low ejaculatory volume is caused by
retrograde ejaculation into the bladder or
obstruction of the vas deferens or the ejaculatory duct
Azoospermia may be seen in
testicular failure or
obstruction of vas deferens
Absence of fructose suggests
seminal vesicles agenesis or obstruction.
A semen that is low in volume with azoospermia and absence of fructose suggests → either obstruction of the ejaculatory ducts or congenital absence of the vas deferens and seminal vesicles.
The anatomy of the vas deferens & seminal vesicles can be investigated by vasography (where contrast medium is injected into the vas deferens) or TRUS (Trans rectal ultrasonography), however TRUS is superior to vasography and is the investigation of choice.
Also know
Semen specimen should be obtained following at least 3 days of abstinence and examined within 1 to 2 hrs. At least 2 specimens are examined several weeks apart.
Colour duplex ultrasonography of the scrotum
Transrectal ultrasonography
Retrograde urethrography
Spermatic venography