**Core Concept**
A patent ductus arteriosus (PDA) in a preterm baby is a congenital heart defect where the ductus arteriosus, a normal fetal blood vessel, fails to close after birth. This condition can lead to excessive blood flow to the lungs, causing pulmonary overcirculation and potential respiratory distress.

**Why the Correct Answer is Right**
In a preterm baby with a PDA, the most common findings include respiratory distress, tachypnea, and a characteristic continuous murmur ("machinery" murmur) heard best at the left upper sternal border. The presence of a PDA in a preterm baby can lead to increased pulmonary blood flow, causing pulmonary overcirculation and potential respiratory distress. This is due to the increased pressure in the pulmonary artery, which in turn causes fluid overload and pulmonary edema.

**Why Each Wrong Option is Incorrect**
* **Option A:** Tachycardia is a common finding in preterm babies with a PDA due to the increased metabolic demand and potential for hypoxia. This is not the least likely finding.
* **Option B:** Bradycardia is not typically associated with a PDA in a preterm baby, as the increased pulmonary blood flow usually leads to tachycardia. This is the least likely finding.
* **Option C:** A characteristic continuous murmur ("machinery" murmur) is a hallmark of a PDA in a preterm baby, heard best at the left upper sternal border. This is not the least likely finding.
* **Option D:** Respiratory distress is a common finding in preterm babies with a PDA due to the increased pulmonary blood flow and potential for pulmonary edema. This is not the least likely finding.

**Clinical Pearl / High-Yield Fact**
In preterm babies with a PDA, early recognition and treatment are crucial to prevent complications such as pulmonary hypertension, pulmonary edema, and chronic lung disease.

**Correct Answer: B. Bradycardia. Bradycardia is the least likely finding in a preterm baby with a PDA.**