**Core Concept**
Speculum examination is a vital diagnostic tool in obstetrics and gynecology used to visualize the cervix and vagina. It helps in identifying potential causes of vaginal discharge, such as infection, cervical incompetence, or membrane rupture. This examination is particularly useful in the context of a pregnant patient presenting with leaking at 34 weeks gestation.

**Why the Correct Answer is Right**
Speculum examination allows for a direct visualization of the cervix and vagina, enabling the healthcare provider to assess the integrity of the fetal membranes and detect any signs of rupture. A speculum examination involves inserting a lubricated speculum into the vagina, which is then gently opened to visualize the cervix and vagina. This examination can help identify the presence of a leak, assess the amount of amniotic fluid, and evaluate the cervix for signs of effacement or dilation.

**Why Each Wrong Option is Incorrect**
**Option B:** USG (Ultrasound) is useful for assessing fetal well-being, gestational age, and amniotic fluid volume but may not directly diagnose a vaginal leak.
**Option C:** Non-stress test is a fetal monitoring test used to assess fetal well-being by measuring fetal heart rate in response to fetal movement. It does not diagnose vaginal leaks.
**Option D:** 3 swab test is used to detect the presence of Group B streptococcal colonization in pregnant women but is not relevant for diagnosing vaginal leaks.

**Clinical Pearl / High-Yield Fact**
Performing a speculum examination in a pregnant patient with suspected membrane rupture should be done gently to avoid causing further disruption of the fetal membranes. It is essential to use a lubricated speculum and to minimize the time spent on the examination to prevent infection.

**✓ Correct Answer: A. Speculum examination**