**Core Concept**
The underlying abnormality in this infant is likely related to a congenital heart defect that involves abnormal blood flow between the chambers of the heart, leading to a murmur and symptoms of heart failure. The presence of a harsh systolic murmur and a thrill suggests a significant turbulent blood flow.

**Why the Correct Answer is Right**
The symptoms of poor feeding, sweating, and difficulty breathing in a previously well infant suggest heart failure. The loud, harsh systolic murmur with a thrill heard at the left sternal border is characteristic of a ventricular septal defect (VSD). A VSD is a defect in the interventricular septum, allowing blood to flow from the left ventricle to the right ventricle. This results in increased flow and pressure in the pulmonary artery, leading to heart failure and symptoms.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not specified, so we cannot provide an explanation.
**Option B:** While patent ductus arteriosus (PDA) is a common congenital heart defect, the description of a harsh systolic murmur with a thrill at the left sternal border is not typical.
**Option C:** Tetralogy of Fallot is a cyanotic congenital heart defect, but it typically presents with a harsh systolic ejection murmur and cyanosis, not a harsh systolic murmur and heart failure symptoms.
**Option D:** While coarctation of the aorta is a congenital heart defect, it typically presents with hypertension in the arms and decreased or delayed pulses in the lower extremities, not a harsh systolic murmur and heart failure symptoms.

**Clinical Pearl / High-Yield Fact**
The "easy going baby" with a sudden onset of symptoms is a classic presentation for a congenital heart defect. Always consider cardiac examination in infants with unexplained poor feeding, sweating, and difficulty breathing.

**Correct Answer:** C. Tetralogy of Fallot