All are tocolytics except :
**Core Concept**
Tocolytics are medications used to suppress premature labor or to delay preterm birth. They act by relaxing the uterine smooth muscle or by inhibiting the production of prostaglandins, which stimulate uterine contractions.
**Why the Correct Answer is Right**
Ritodrine is a beta-2 adrenergic agonist that works by stimulating the beta-2 receptors in the uterus, leading to relaxation of the uterine smooth muscle and suppression of uterine contractions. This action is similar to other tocolytics like terbutaline and salbutamol.
**Why Each Wrong Option is Incorrect**
**Option A:** Oxytocin is actually a uterotonic agent, which means it stimulates uterine contractions rather than suppressing them. It is commonly used to induce or augment labor.
**Option B:** Magnesium sulfate is a commonly used tocolytic, acting by relaxing the uterine smooth muscle and reducing the release of acetylcholine, which stimulates uterine contractions.
**Option C:** Nifedipine is a calcium channel blocker that relaxes the uterine smooth muscle by inhibiting the influx of calcium ions, which are necessary for muscle contraction.
**Clinical Pearl / High-Yield Fact**
Remember that tocolytics are used to delay preterm birth, but they do not cure the underlying cause of preterm labor. A thorough evaluation of the patient's medical history and risk factors is essential to determine the best course of treatment.
**Correct Answer:** A. Oxytocin.