**Core Concept**
The child's presentation of sudden onset altered sensorium with severely elevated blood pressure (hypertension) is suggestive of a hypertensive emergency, specifically malignant hypertension, which can lead to acute end-organ damage.

**Why the Correct Answer is Right**
Malignant hypertension is characterized by a systolic blood pressure greater than 180 mmHg or a diastolic blood pressure greater than 120 mmHg. In children, such severe hypertension is often due to underlying renal disease, such as renal scarring or glomerulonephritis. The sudden onset of altered sensorium in this child is likely due to cerebral edema or stroke secondary to the severe hypertension. The renal disease may also lead to fluid overload, causing pulmonary edema or acute kidney injury.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not address the child's severe hypertension and altered sensorium. While seizures can cause altered sensorium, they do not explain the child's elevated blood pressure.
* **Option B:** This option is incorrect because it does not take into account the child's severe hypertension. While infection can cause altered sensorium, it is unlikely to result in such high blood pressure readings.
* **Option C:** This option is incorrect because it is not a direct cause of the child's altered sensorium. While cardiac disease can lead to hypertension, it is not the most likely cause in this scenario.

**Clinical Pearl / High-Yield Fact**
In pediatric patients, malignant hypertension can be caused by underlying renal disease, such as renal scarring or glomerulonephritis. It is essential to investigate for underlying renal disease in children presenting with severe hypertension.

**Correct Answer:** C. Hypertensive emergency due to renal disease.