**Core Concept**
Ritodrine is a beta-2 adrenergic agonist used to inhibit uterine contractility, thereby delaying preterm labor. However, its use is associated with cardiovascular side effects, including pulmonary edema, due to its ability to increase cardiac output and reduce systemic vascular resistance.

**Why the Correct Answer is Right**
Ritodrine acts on the beta-2 adrenergic receptors in the uterus, causing relaxation of the uterine smooth muscle and inhibition of contractility. This is achieved through activation of the adenylate cyclase pathway, leading to increased levels of cyclic AMP (cAMP) and subsequent relaxation of the uterine muscle cells. The increased cardiac output and decreased systemic vascular resistance caused by ritodrine can lead to pulmonary edema, particularly in susceptible individuals.

**Why Each Wrong Option is Incorrect**
**Option B:** Nifedipine is a calcium channel blocker that inhibits uterine contractility by reducing the influx of calcium ions into the smooth muscle cells. However, it does not cause pulmonary edema as a side effect.
**Option C:** Indomethacin is a nonsteroidal anti-inflammatory drug (NSAID) that inhibits prostaglandin synthesis, which in turn reduces uterine contractility. It does not cause pulmonary edema.
**Option D:** Atosiban is an oxytocin receptor antagonist that inhibits uterine contractility by blocking the action of oxytocin on the uterine muscle cells. It does not cause pulmonary edema as a side effect.

**Clinical Pearl / High-Yield Fact**
Beta-2 adrenergic agonists like ritodrine can cause maternal hypotension and tachycardia, which can compromise fetal well-being. Therefore, their use should be carefully monitored in pregnant women.

✓ Correct Answer: A. Ritodrine