**Core Concept**
Ritodrine is a selective beta-2 adrenergic agonist used in obstetrics to suppress preterm labor by relaxing uterine smooth muscle. It acts via stimulation of beta-2 receptors in the myometrium, leading to reduced uterine contractility.

**Why the Correct Answer is Right**
Ritodrine is specifically indicated for the management of preterm labor due to its selective action on beta-2 adrenergic receptors in the uterus. It induces smooth muscle relaxation without significant cardiovascular side effects, making it effective in arresting contractions. It is the only option among the listed drugs that has been clinically validated for this purpose in preterm labor.

**Why Each Wrong Option is Incorrect**
Option B: Isoprenaline is a non-selective beta-1 and beta-2 agonist with significant cardiovascular effects, including tachycardia and hypotension, making it unsuitable for use in labor.
Option C: Salbutamol is a beta-2 agonist used primarily in asthma, not in labor, and lacks the uterine-specific action needed for labor arrest.
Option D: Terbutaline is also a beta-2 agonist used in asthma and preterm labor, but it is less preferred than ritodrine due to higher risk of tachyphylaxis and side effects; it is not the first-line agent.

**Clinical Pearl / High-Yield Fact**
Ritodrine is the only adrenergic drug specifically approved and preferred for arresting preterm labor due to its uterine selectivity and proven efficacy—though it is now less commonly used due to safety concerns. Always monitor for maternal tachycardia and fetal distress.

✓ Correct Answer: A. Ritodrine