**Core Concept**
The question is testing the understanding of the diagnostic approach to azoospermia, a condition where the semen analysis reveals the absence of sperm. Azoospermia can be primary (present at birth) or secondary (develops after puberty). The normal FSH and testosterone levels in this case suggest that the hypothalamic-pituitary-gonadal axis is intact, making it less likely to be a case of hypogonadotropic hypogonadism.

**Why the Correct Answer is Right**
The most probable cause of azoospermia in this case is obstructive azoospermia, which is due to a blockage in the reproductive tract that prevents sperm from reaching the semen. The presence of normal FSH and testosterone levels indicates that the testes are functioning properly and are producing adequate amounts of hormones. However, the absence of sperm in the semen suggests that there is an obstruction in the epididymis, vas deferens, or ejaculatory ducts, which are the structures responsible for transporting sperm from the testes to the semen.

**Why Each Wrong Option is Incorrect**
* **Option A:** Klinefelter syndrome is a genetic condition that affects males and is characterized by the presence of an extra X chromosome (XXY). It is associated with hypogonadism, which is not consistent with the normal testosterone levels in this case.
* **Option B:** Testicular failure, also known as non-obstructive azoospermia, is characterized by the absence of sperm in the semen due to a problem with sperm production in the testes. However, the normal FSH and testosterone levels in this case suggest that the testes are functioning properly.
* **Option C:** This option is not provided, so it cannot be evaluated.

**Clinical Pearl / High-Yield Fact**
In cases of azoospermia, it is essential to perform a detailed history and physical examination, as well as imaging studies such as ultrasound or CT scans, to identify any potential obstructions in the reproductive tract.

**Correct Answer: D. Obstructive azoospermia.**