**Core Concept**
The premature rupture of membranes (PROM) in a pregnant woman can be confirmed through a specific clinical examination that assesses the presence of amniotic fluid in the vagina. This is crucial for diagnosing and managing preterm labor.

**Why the Correct Answer is Right**
The correct examination to confirm premature rupture of membranes is the **Nitrazine test** or the **Fern test**. However, the most widely accepted method is the **Nitrazine test**. The Nitrazine test involves dipping a pH paper into the vaginal discharge. A pH reading of less than 4.5 is indicative of the presence of amniotic fluid, which has a pH of approximately 7.0. This is because amniotic fluid has a higher pH compared to the vaginal secretions. The presence of amniotic fluid is confirmed when the pH paper turns blue or purple.

**Why Each Wrong Option is Incorrect**
* **Option A:** The **Nitrazine test** is not the same as the **Fern test**, although both are used to assess the presence of amniotic fluid. However, the Fern test involves examining the discharge under a microscope for fern-like patterns, which are indicative of amniotic fluid.
* **Option B:** **Speculum examination** alone is not sufficient to confirm premature rupture of membranes, as it only visualizes the discharge and does not assess its pH or composition.
* **Option C:** **Ultrasonography** is not used to confirm premature rupture of membranes, although it may be used to assess the fetal well-being and the amount of amniotic fluid present.

**Clinical Pearl / High-Yield Fact**
In cases of suspected premature rupture of membranes, it's essential to perform the Nitrazine test as soon as possible to confirm the diagnosis. A positive result can help guide further management, including the administration of antibiotics and the initiation of labor.

**Correct Answer:** C.