**Core Concept**
The earliest diagnostic test of pregnancy is based on the detection of human chorionic gonadotropin (hCG) in the urine or blood. hCG is a hormone produced by the placenta shortly after implantation of the fertilized ovum.

**Why the Correct Answer is Right**
The earliest diagnostic test of pregnancy is the detection of hCG, which is produced by the placenta as early as 6-8 days after fertilization. This hormone is essential for maintaining the corpus luteum, which in turn supports the early development of the embryo. The hCG test is highly sensitive and can detect even small amounts of the hormone in the urine or blood, making it the earliest diagnostic test of pregnancy.

**Why Each Wrong Option is Incorrect**
**Option A:**
This option is not relevant to the earliest diagnostic test of pregnancy.

**Option B:**
This option is incorrect because it is not the earliest diagnostic test of pregnancy.

**Option C:**
This option is incorrect because it is not a diagnostic test of pregnancy.

**Clinical Pearl / High-Yield Fact**
The earliest diagnostic test of pregnancy can detect hCG levels as low as 6-8 days after fertilization, making it an essential tool in confirming pregnancy in the early stages.

**Correct Answer:** A. Urine hCG test.