**Core Concept**
The patient's presentation of azoospermia, normal testicular size, and normal FSH and testosterone levels points to a condition where sperm production is impaired despite normal hormonal regulation. This scenario suggests a problem with the epididymis or vas deferens, which are involved in sperm maturation and transport, rather than a hormonal or testicular issue.

**Why the Correct Answer is Right**
The absence of sperm in the ejaculate despite normal hormonal regulation and testicular size suggests a problem with the epididymal or vasal system. The most common cause of this condition is congenital bilateral absence of the vas deferens (CBAVD), which is often associated with cystic fibrosis. In CBAVD, the epididymis and vas deferens are absent or severely impaired, leading to obstructive azoospermia. This condition is characterized by the absence of vasal obstruction on imaging studies and normal FSH and testosterone levels.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because varicocele is a condition where the veins of the scrotum are enlarged, which can affect sperm quality and quantity but does not typically cause azoospermia with normal FSH and testosterone levels.

**Option B:** This option is incorrect because Klinefelter syndrome is a condition characterized by an extra X chromosome in males, leading to hypogonadism and low testosterone levels, which is not consistent with the patient's presentation of normal testosterone levels.

**Option C:** This option is incorrect because obstructive azoospermia due to vasal obstruction would typically present with elevated FSH levels, as the testes are not receiving feedback from the ejaculated sperm. However, in this case, the FSH levels are normal.

**Clinical Pearl / High-Yield Fact**
Azoospermia with normal FSH and testosterone levels should prompt an investigation into the epididymal or vasal system, as this is often indicative of obstructive azoospermia due to a problem with the epididymis or vas deferens.

**Correct Answer:** C.