**Core Concept**
The child in question likely has a patent ductus arteriosus (PDA), a congenital heart defect characterized by the failure of the ductus arteriosus to close after birth. This results in a persistent left-to-right shunt between the aorta and the pulmonary artery, leading to respiratory distress and other cardiac complications.

**Why the Correct Answer is Right**
The clinical presentation of increased pulse pressure and a pansystolic murmur best heard in the 2nd left intercostal space is typical of PDA. The murmur is caused by the turbulent blood flow through the PDA, which creates a characteristic "machinery" or "whooshing" sound. In premature infants, PDA is often associated with respiratory distress due to excessive pulmonary blood flow. The increased pulse pressure is a result of the increased aortic runoff into the pulmonary artery.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided as there are only 3 options given in the problem.
**Option B:** This option is incorrect as it is not related to the clinical presentation described.
**Option C:** This option is incorrect as it does not describe the typical murmur and increased pulse pressure associated with PDA.

**Clinical Pearl / High-Yield Fact**
Premature infants with PDA often present with respiratory distress, failure to thrive, and a characteristic "machinery" murmur. Early diagnosis and closure of the PDA are crucial to prevent complications such as pulmonary hypertension and cardiac failure.

**Correct Answer: D. Patent Ductus Arteriosus**