**Core Concept**
Fetal exsanguination refers to the rapid loss of fetal blood, which can be life-threatening. Vasa previa is a condition where fetal blood vessels are located in the membranes covering the cervix, below the presenting fetal part.

**Why the Correct Answer is Right**
In vasa previa, the fetal blood vessels are unsupported by the placenta or umbilical cord, making them susceptible to rupture during labor, particularly with rupture of membranes or with membrane rupture during labor. This can lead to massive fetal hemorrhage and exsanguination. The fetal blood vessels are usually covered by a thin membrane called Wharton's jelly, which provides protection against rupture. However, in vasa previa, this protective covering is absent or compromised, increasing the risk of blood vessel rupture.

**Why Each Wrong Option is Incorrect**
**Option B:** Placenta previa refers to the abnormal implantation of the placenta over or near the internal cervical os. While placenta previa can cause bleeding, it does not directly cause fetal exsanguination. Bleeding in placenta previa typically occurs due to separation of the placenta from the uterus, whereas vasa previa involves rupture of fetal blood vessels.

**Option C:** Polyhydramnios refers to an excessive amount of amniotic fluid. While polyhydramnios can cause complications during pregnancy, such as preterm labor and fetal distress, it does not directly cause fetal exsanguination.

**Option D:** Oligohydramnios refers to a low amount of amniotic fluid. Like polyhydramnios, oligohydramnios can cause complications during pregnancy, but it does not directly cause fetal exsanguination.

**Clinical Pearl / High-Yield Fact**
When suspecting vasa previa, clinical features may include vaginal bleeding in the second or third trimester, often accompanied by fetal distress or bradycardia. A key diagnostic clue is the presence of fetal blood vessels crossing over the internal cervical os, visible on ultrasound.

**✓ Correct Answer: A. Vasa previa**