**Core Concept**
Ritodrine is a beta-2 adrenergic agonist used to inhibit uterine contractility, thereby maintaining pregnancy in preterm labor. However, its administration is associated with cardiovascular side effects, including pulmonary edema.

**Why the Correct Answer is Right**
Ritodrine works by stimulating beta-2 adrenergic receptors in the uterus, leading to relaxation of uterine smooth muscle and inhibition of contractions. This effect is mediated by increased levels of cyclic AMP (cAMP) within the uterine muscle cells. As a beta-2 adrenergic agonist, ritodrine also causes vasodilation, which can lead to a decrease in systemic vascular resistance and, consequently, a decrease in blood pressure. This can compromise cardiac function and lead to pulmonary edema, particularly in patients with pre-existing cardiac disease.

**Why Each Wrong Option is Incorrect**
**Option B:** Nifedipine is a calcium channel blocker that inhibits uterine contractility by reducing calcium influx into the uterine smooth muscle cells. However, it does not cause pulmonary edema as a primary side effect.
**Option C:** Indomethacin is a nonsteroidal anti-inflammatory drug (NSAID) that inhibits uterine contractility by blocking the synthesis of prostaglandins, which are key mediators of uterine contractions. NSAIDs like indomethacin are not associated with pulmonary edema.
**Option D:** Atosiban is an oxytocin receptor antagonist that inhibits uterine contractility by blocking the action of oxytocin at the uterine receptor. Atosiban does not cause pulmonary edema as a primary side effect.

**Clinical Pearl / High-Yield Fact**
Beta-2 adrenergic agonists like ritodrine can cause pulmonary edema due to their vasodilatory effects, which can compromise cardiac function, particularly in patients with pre-existing cardiac disease.

**✓ Correct Answer: A. Ritodrine**