## Core Concept
The Gravindex test, also known as the Gravindex test, is a type of immunological test used for detecting pregnancy. It works on the principle of detecting human chorionic gonadotropin (hCG) in the urine, which is a hormone produced during pregnancy.

## Why the Correct Answer is Right
The correct answer, **2-3 days after implantation**, is right because implantation of the blastocyst into the uterine lining typically occurs 6-10 days after fertilization. The hCG levels start rising significantly after implantation and can be detected in the blood and urine around 2-3 days after implantation. This timeframe allows for early detection of pregnancy through tests that measure hCG levels.

## Why Each Wrong Option is Incorrect
- **Option A: Immediately after fertilization** - This is incorrect because hCG levels are not significantly elevated immediately after fertilization. It takes time for the embryo to implant and for hCG production to increase.
- **Option B: 1 day after implantation** - This is too early for most pregnancy tests to detect hCG reliably. It takes a couple of days after implantation for hCG levels to rise sufficiently.
- **Option D: 1 week after missed period** - While pregnancy tests can certainly detect pregnancy at this point, the Gravindex test is specifically notable for its ability to detect pregnancy earlier, around 2-3 days after implantation.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the sensitivity of pregnancy tests, including the Gravindex test, can vary. Tests that can detect pregnancy early often have higher sensitivity to hCG. This is crucial for early detection and management of pregnancy.

## Correct Answer: C. 2-3 days after implantation