**Core Concept**
Absence of sinus arrhythmia in children is a characteristic feature of certain congenital heart diseases. Sinus arrhythmia is a normal variation in heart rate that occurs with breathing, with a decrease in heart rate during inspiration and an increase during expiration. In congenital heart disease, this normal variation can be disrupted due to abnormal hemodynamics.

**Why the Correct Answer is Right**
Tetralogy of Fallot (TOF) is a congenital heart disease characterized by four components: ventricular septal defect, pulmonary stenosis, right ventricular hypertrophy, and an overriding aorta. In TOF, the presence of pulmonary stenosis leads to a fixed cardiac output, which in turn abolishes the normal variation in heart rate with breathing, resulting in absence of sinus arrhythmia. This is due to the fixed cardiac output, which does not change with breathing, unlike in normal individuals.

**Why Each Wrong Option is Incorrect**
**Option A:** Transposition of the great arteries does not typically present with absence of sinus arrhythmia, as the heart rate variation with breathing is usually preserved.
**Option B:** Tricuspid atresia is characterized by a fixed cardiac output, but it does not typically present with absence of sinus arrhythmia, as the heart rate variation with breathing is usually preserved.
**Option C:** Ebstein's anomaly is characterized by abnormalities in the tricuspid valve and right ventricle, but it does not typically present with absence of sinus arrhythmia, as the heart rate variation with breathing is usually preserved.

**Clinical Pearl / High-Yield Fact**
A key feature of Tetralogy of Fallot is the presence of a harsh ejection systolic murmur due to pulmonary stenosis, which is an important clue to the diagnosis.

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