**Core Concept**
The diagnosis of premature rupture of membranes (PROM) involves confirming the presence of amniotic fluid in the vagina. This can be challenging due to vaginal discharge and bleeding in pregnancy. A reliable examination is crucial to determine the best course of action.

**Why the Correct Answer is Right**
The most reliable examination to confirm PROM is the **Nitrazine test**. This test involves dipping a pH indicator strip into the vaginal discharge. Amniotic fluid is alkaline, with a pH of 7-8, whereas normal vaginal discharge is acidic with a pH of 3.8-4.5. The Nitrazine test is a simple, non-invasive, and rapid method to confirm the presence of amniotic fluid.

**Why Each Wrong Option is Incorrect**
**Option A:** Fetal heart rate monitoring is not a direct method to confirm PROM. It can be used to assess fetal well-being but does not provide a definitive diagnosis of PROM.
**Option B:** Ultrasound can be used to evaluate the amniotic fluid index and detect oligohydramnios, but it is not a reliable method to confirm PROM.
**Option C:** The fern test, also known as the microscope test, involves examining the vaginal discharge under a microscope for the presence of fern-like patterns. However, this test is less reliable than the Nitrazine test and can be affected by various factors.

**Clinical Pearl / High-Yield Fact**
The Nitrazine test can also be used to differentiate between amniotic fluid and other types of vaginal discharge. However, a positive result does not confirm PROM in cases of multiple gestations or polyhydramnios.

**Correct Answer:** C. The fern test is an alternative method to confirm PROM, but the Nitrazine test is more reliable.