**Core Concept**
Prolonged pregnancy, also known as post-term pregnancy, is a condition where the gestational age exceeds 42 weeks. However, in this scenario, the patient is at 30 weeks gestation, which is considered an early gestation. The key concept here is oligohydramnios, a condition characterized by decreased amniotic fluid volume.

**Why the Correct Answer is Right**
Oligohydramnios can be caused by various factors, including fetal renal anomalies or obstruction, which prevent the normal production of amniotic fluid. In this case, the amniotic fluid index (AFI) of 4cm, which is significantly lower than the normal range, suggests oligohydramnios. The fetus's kidneys play a crucial role in producing amniotic fluid through the process of fetal urination. If the fetal kidneys are not functioning properly, it can lead to decreased amniotic fluid production.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the scenario, as it does not relate to the decreased amniotic fluid volume.

**Option B:** Polyhydramnios is a condition characterized by an excessive amount of amniotic fluid, which is the opposite of oligohydramnios.

**Option C:** Fetal growth restriction (FGR) is a condition where the fetus does not grow at a normal rate, but it is not directly related to the decreased amniotic fluid volume.

**Option D:** Premature rupture of membranes is a condition where the membranes surrounding the fetus rupture before 37 weeks of gestation, which is not directly related to the decreased amniotic fluid volume.

**Clinical Pearl / High-Yield Fact**
A useful mnemonic to remember the causes of decreased amniotic fluid is "FROG": Fetal Renal Obstruction, Growth restriction, Oligohydramnios, and Gastroschisis.

**Correct Answer:** C. Fetal growth restriction