**Core Concept**
Fetal anemia is a condition characterized by a decrease in fetal red blood cell mass, leading to a decrease in fetal hemoglobin levels. This condition can occur due to various causes such as alloimmunization, twin-to-twin transfusion syndrome, or congenital heart disease. The detection of fetal anemia is crucial for timely intervention to prevent fetal compromise.

**Why the Correct Answer is Right**
Fetal anemia can be detected using Doppler ultrasound by evaluating the middle cerebral artery (MCA) peak systolic velocity (PSV). In a fetus with anemia, there is a compensatory increase in cardiac output, which leads to an increase in MCA PSV. This is because the anemic fetus requires more blood flow to the brain to maintain adequate oxygen delivery. The MCA PSV is a sensitive indicator of fetal anemia, and an increase in this value is a critical marker for diagnosing this condition.

**Why Each Wrong Option is Incorrect**
**Option A:** While fetal growth restriction (FGR) can be associated with fetal anemia, it is not a direct indicator of anemia. FGR can be caused by various factors, including placental insufficiency, and is not specific to anemia.
**Option B:** The umbilical artery (UA) Doppler is used to evaluate placental function and fetal well-being, but it is not a specific indicator of fetal anemia.
**Option C:** The ductus venosus (DV) Doppler is used to evaluate fetal cardiac function, but it is not a direct indicator of fetal anemia.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that an MCA PSV of > 1.5 MoM (multiple of the median) is considered abnormal and suggestive of fetal anemia. This is a critical threshold for diagnosing fetal anemia and warrants further investigation and management.

**Correct Answer: C. Ductus venosus Doppler is used to evaluate fetal cardiac function but is not a direct indicator of fetal anemia.**