**Core Concept**
Patent ductus arteriosus (PDA) is a congenital heart defect characterized by the persistent patency of the ductus arteriosus, a vascular shunt between the aorta and the pulmonary artery. This condition leads to excessive blood flow to the lungs, resulting in pulmonary overcirculation and potential complications.

**Why the Correct Answer is Right**
The persistent left-to-right shunt in PDA causes increased pulmonary blood flow, leading to pulmonary hypertension and eventually right ventricular hypertrophy. The increased volume load on the left ventricle can also result in left ventricular hypertrophy. Over time, this can lead to pulmonary oedema, heart failure, and potentially life-threatening complications.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not directly related to the consequences of large PDA. While pulmonary hypertension can be a long-term complication, it is not the primary effect of the shunt.

**Option B:** This option is incorrect as PDA does not typically lead to decreased cardiac output. In fact, the shunt can result in increased cardiac output due to the increased volume load on the left ventricle.

**Option C:** This option is incorrect as PDA is not typically associated with decreased pulmonary blood flow. In fact, the shunt leads to increased pulmonary blood flow, which can result in pulmonary overcirculation and complications.

**Clinical Pearl / High-Yield Fact**
A key clinical feature of large PDA is the presence of a continuous murmur ("machinery" murmur) best heard at the left upper sternal border, which is a result of the shunt between the aorta and the pulmonary artery.

**Correct Answer:** C. Pulmonary overcirculation and potential complications