## Core Concept
Fetal blood is differentiated from maternal blood primarily through the unique characteristics of fetal hemoglobin and the placental structure. The main method of differentiation involves the presence of **fetal hemoglobin (HbF)**, which has a different structure and function compared to adult hemoglobin (HbA).

## Why the Correct Answer is Right
The correct answer, **Kleihauer test**, is a method used to detect fetal hemoglobin in maternal blood. This test takes advantage of the fact that **fetal hemoglobin (HbF) is more resistant to acid elution** than adult hemoglobin. When a blood smear is treated with acid, adult hemoglobin is eluted out of the red blood cells, while fetal hemoglobin remains. This allows for the identification and quantification of fetal red blood cells in the maternal circulation. The Kleihauer test is particularly useful in determining the presence and amount of fetal-maternal hemorrhage.

## Why Each Wrong Option is Incorrect
**Option A:** This option is incorrect because it does not correspond to a recognized method for differentiating fetal blood from maternal blood.

**Option B:** This option is incorrect as it does not represent a valid method for distinguishing between fetal and maternal blood.

**Option C:** While **Electrophoresis** can be used to differentiate between types of hemoglobin, including fetal hemoglobin, it is not the specific test referred to in the context of quickly identifying fetal blood cells in maternal circulation.

## Clinical Pearl / High-Yield Fact
A key clinical application of the Kleihauer test is in **RhD alloimmunization**, where it is used to assess the need for additional Rh immune globulin administration to prevent isoimmunization in Rh-negative mothers. This test helps in estimating the volume of fetal-maternal hemorrhage.

## Correct Answer: B. Kleihauer test