An 18-year-old man is evaluated in the emergency department for abdominal cramping and bloody diarrhea of 6 days' duration. Medical history is unremarkable, and he takes no medications. On physical examination, the temperature is 37.0 °C (98.6 °F), blood pressure is 98/60 mm Hg, pulse rate is 100/min, and respiration rate is 16/min. Oxygen saturation is normal while breathing ambient air. His abdomen is tender, without guarding or organomegaly. The examination is otherwise unremarkable.

Laboratory studies:
- Haptoglobin: Undetectable
- Hemoglobin: 6.1 g/dL (61 g/L)
- Leukocyte count: 6800/mL (6.8 × 10^9/L)
- Platelet count: 37,000/mL (37 × 10^9/L)
- Reticulocyte count: 9.8%
- Creatinine: 3.6 mg/dL (318 mmol/L)

Urinalysis:
- 3+ blood
- 3+ protein
- 0-2 erythrocytes/hpf
- 0-2 leukocytes/hpf
- Several granular casts

The peripheral blood smear shows schistocytes and scant platelets without clumps. The direct antiglobulin (Coombs) test is negative.

What is the most likely diagnosis?

1) Atypical hemolytic uremic syndrome
2) Hemolytic uremic syndrome
3) Immune hemolytic anemia and thrombocytopenia
4) Rapidly progressive glomerulonephritis