**Core Concept**
Preterm labor is a significant risk factor for neonatal morbidity and mortality, especially in women with a history of preterm birth. The management of preterm labor involves identifying the cause, which may be infection, cervical insufficiency, or other factors. In this scenario, the patient is at 35 weeks gestation and experiencing uterine contractions, suggesting preterm labor.

**Why the Correct Answer is Right**
The correct answer is **B. Vaginal swab culture**. This is because asymptomatic bacterial vaginosis and group B streptococcal colonization are common in pregnant women and can trigger preterm labor. A vaginal swab culture is essential to identify the causative organism, which guides the management of preterm labor. The American College of Obstetricians and Gynecologists (ACOG) recommends screening for group B streptococcus at 35-37 weeks of gestation. Early identification of infection allows for prompt antibiotic treatment, which may prevent preterm birth.

**Why Each Wrong Option is Incorrect**
**Option A:** Administration of betamethasone (12 mg) is indicated for fetal lung maturity in cases of preterm labor with fetal distress or when delivery is imminent within 7 days. However, in this scenario, there is no fetal distress, and the patient is not yet at term.

**Option C:** Tocolytic therapy (e.g., nifedipine, magnesium sulfate) is used to delay preterm birth, but it is not a first-line approach in this scenario. The patient's membranes are not ruptured, and there is no fetal distress, making tocolytic therapy less necessary.

**Option D:** Cervical cerclage is indicated for cervical insufficiency in women with a history of cervical insufficiency or a short cervix. However, in this scenario, the patient's membranes are not ruptured, and there is no evidence of cervical insufficiency.

**Clinical Pearl / High-Yield Fact**
A high index of suspicion for infection is crucial in managing preterm labor. A vaginal swab culture should be performed in all pregnant women at risk for preterm labor to guide antibiotic treatment and prevent preterm birth.

**Correct Answer:**
✓ Correct Answer: B. Vaginal swab culture