**Core Concept:**
The question is asking about the diagnosis of a child with high blood pressure, elevated kidney function tests, and significant urinary abnormalities. The correct diagnosis requires understanding of the pathophysiology of essential hypertension and its complications in children, the role of urine analysis in diagnosing acute kidney injury, and the significance of pus cells, red blood cells, and proteinuria.

**Why the Correct Answer is Right:**
In this case, the correct answer is **D.** Acute kidney injury (AKI) is the most likely diagnosis, considering the following factors:

1. **Pathophysiology:** Essential hypertension in children, also known as primary hypertension, results from an abnormal increase in blood pressure without a known cause. In the presented case, the blood pressure is high, which indicates the presence of essential hypertension.

2. **Complications:** Essential hypertension can lead to various complications, including kidney damage, due to the increased workload on the kidneys. This increased workload can result in acute kidney injury.

3. **Urinalysis findings:** The presence of pus cells, red blood cells, and proteinuria in the urine sample indicates acute tubular necrosis, which is a common feature of AKI caused by essential hypertension.

4. **Absence of past history:** The child has no significant past history of similar complaints, ruling out chronic conditions like glomerulonephritis and polycystic kidney disease.

**Why Each Wrong Option is Incorrect:**

A. **Option A:** Essential hypertension is unlikely to cause hypertension in childhood, as the question states the child's hypertension is of 8 years duration, which is too short a duration for essential hypertension.

B. **Option B:** The elevated urea and creatinine levels could be indicative of AKI, but the urinalysis findings and absence of past history make this option less likely.

C. **Option C:** Chronic renal disease is another possible cause of hypertension and elevated kidney function tests, but the urinalysis findings and absence of past history make this option less likely.

**Clinical Pearl:**
Acute kidney injury (AKI) is a significant clinical entity in children with essential hypertension, as it can lead to severe complications, including the need for dialysis or kidney transplantation. In this case, a detailed history, clinical examination, and urinalysis findings support the diagnosis of AKI due to essential hypertension. Therefore, the correct answer is **D.** Acute kidney injury as a complication of essential hypertension in children is a crucial aspect of clinical practice and should be kept in mind while treating children with hypertension.