**Core Concept**
Azoospermia refers to the absence of sperm in the ejaculate, which can be due to various causes including testicular failure or obstruction of the Vas deferens. The underlying principle being tested is the ability to differentiate between these two main categories of azoospermia using diagnostic tests.

**Why the Correct Answer is Right**
The correct diagnostic test to distinguish between testicular failure and obstruction of the Vas deferens involves assessing the presence or absence of sperm in the seminal fluid and evaluating the hormonal and physical characteristics of the testes.

**Why Each Wrong Option is Incorrect**
**Option A:**
**Option B:**
**Option C:**
Since the question is incomplete and the answer choices are missing, we cannot provide specific reasons for why each option is incorrect.

**Clinical Pearl / High-Yield Fact**
A key point in diagnosing azoospermia is to determine if the issue is with sperm production (testicular failure) or sperm transport (obstruction), which guides treatment options.

**Correct Answer:** D. FSH level