**Core Concept**
The fetomaternal hemorrhage (FMH) test is essential to assess the amount of fetal blood that has entered the maternal circulation during pregnancy or delivery, particularly in Rh-negative mothers to prevent isoimmunization. Accurate estimation of FMH is crucial to determine the need for Rh immunoglobulin (RhIg) administration to prevent Rh alloimmunization.

**Why the Correct Answer is Right**
The Kleihauer-Betke acid elution test is a sensitive and specific test for detecting and quantifying fetomaternal hemorrhage. This test involves acid elution of fetal hemoglobin from maternal red blood cells, allowing for the visualization of fetal cells under a microscope. The test is based on the principle that fetal hemoglobin is more resistant to acid elution than adult hemoglobin, enabling the identification and quantification of fetal cells in maternal blood. The percentage of fetal cells in maternal blood directly correlates with the volume of fetomaternal hemorrhage.

**Why Each Wrong Option is Incorrect**
**Option A:** The Rosette test is an older method for detecting FMH but is less sensitive and specific than the Kleihauer-Betke test. It involves the formation of rosettes of red blood cells that are thought to represent fetal cells, but this method is not reliable for quantifying FMH.

**Option C:** The indirect Coombs test detects the presence of antibodies against red blood cells but does not quantify fetomaternal hemorrhage. It is used to diagnose isoimmunization but not to estimate the size of FMH.

**Option D:** Middle cerebral artery peak systolic velocity (MCA-PSV) is a Doppler ultrasound parameter used to assess fetal anemia, not to estimate fetomaternal hemorrhage. While MCA-PSV can indicate fetal anemia, it does not directly measure the volume of FMH.

**Clinical Pearl / High-Yield Fact**
In cases of suspected fetomaternal hemorrhage, it is essential to administer Rh immunoglobulin (RhIg) within 72 hours of the event to prevent Rh alloimmunization. The dose of RhIg required is directly related to the volume of FMH, which can be estimated using the Kleihauer-Betke test.

**✓ Correct Answer: B. Kleihauer-Betke acid elution test**