**Core Concept**
Nephrotic syndrome, characterized by heavy proteinuria, hypoalbuminemia, and edema, can lead to acute kidney injury (AKI) due to various mechanisms. One such mechanism is the activation of the renin-angiotensin-aldosterone system (RAAS), which increases sodium reabsorption and water retention, ultimately leading to hypervolemia and hypertension. Additionally, the loss of albumin in the urine can cause a decrease in oncotic pressure, leading to decreased glomerular filtration rate (GFR) and subsequent AKI.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of nephrotic syndrome and AKI. In this scenario, the child's presentation with AKI following an episode of diarrhoea suggests a possible link to the activation of the RAAS system. The loss of albumin and the activation of RAAS can lead to increased sodium reabsorption, water retention, and subsequent hypertension, which can cause AKI. This is a common mechanism in patients with nephrotic syndrome.

**Why Each Wrong Option is Incorrect**
**Option A:** This option may seem plausible as acute kidney injury can be caused by various factors, including dehydration and hypovolemia. However, in the context of nephrotic syndrome, the primary mechanism is not dehydration but rather the activation of the RAAS system and the loss of albumin in the urine.

**Option B:** This option is incorrect because acute kidney injury can indeed be caused by hypercalcemia, which can lead to nephrocalcinosis and subsequent AKI. However, in the context of nephrotic syndrome, this is not the primary mechanism.

**Option C:** This option is incorrect because acute kidney injury can be caused by various medications, including nonsteroidal anti-inflammatory drugs (NSAIDs). However, in the context of nephrotic syndrome, this is not the primary mechanism.

**Option D:** This option is incorrect because acute kidney injury can be caused by various infections, including pyelonephritis. However, in the context of nephrotic syndrome, this is not the primary mechanism.

**Clinical Pearl / High-Yield Fact**
In patients with nephrotic syndrome, the activation of the RAAS system and the loss of albumin in the urine can lead to AKI. It is essential to consider these mechanisms when managing patients with nephrotic syndrome and AKI.

**Correct Answer:** D.