**Core Concept**
The child's presentation of reduced urine output, petechial spots, and a history of diarrhea suggests a diagnosis related to a systemic disorder affecting multiple organ systems. The laboratory findings of anemia, leukocytosis, and thrombocytopenia point towards a hematologic or renal disorder.

**Why the Correct Answer is Right**
The peripheral smear is crucial in making the diagnosis. The presence of schistocytes (fragmented red blood cells) and thrombocytopenia with a normal platelet morphology suggest a diagnosis of Thrombotic Thrombocytopenic Purpura (TTP). TTP is characterized by the formation of microthrombi in small blood vessels, leading to microangiopathic hemolytic anemia, thrombocytopenia, and renal failure. The history of diarrhea 2 weeks prior to the current presentation may be related to an infection with E. coli O157:H7, which can trigger TTP.

**Why Each Wrong Option is Incorrect**
**Option A:** Hemolytic Uremic Syndrome (HUS) is a possibility, but the presence of schistocytes and thrombocytopenia with normal platelet morphology makes TTP a more likely diagnosis.

**Option B:** Disseminated Intravascular Coagulation (DIC) is characterized by the consumption of platelets and coagulation factors, leading to bleeding and thrombosis. However, the peripheral smear in DIC typically shows a mixture of both platelet and fibrin degradation products, which was not mentioned in the question stem.

**Option C:** Acute Kidney Injury (AKI) is a non-specific diagnosis that can be seen in TTP, but it does not explain the petechial spots and the laboratory findings.

**Option D:** Meningococcemia can present with petechial spots, but it is typically associated with a high fever and a more acute presentation.

**Clinical Pearl / High-Yield Fact**
TTP is a medical emergency that requires prompt recognition and treatment with plasma exchange to remove the autoantibodies against ADAMTS13, a von Willebrand factor-cleaving protease.

**Correct Answer:** C.