What is the most likely diagnosis for a 10-year-old patient presenting with polyuria (excessive urine production), polydipsia (excessive thirst), an elevated Hemoglobin A1c (HbA1c) level, and a low C-peptide level?

Type 1 Diabetes Mellitus

The most likely diagnosis is Type 1 diabetes mellitus (Option A), as the profoundly low C-peptide level of 0.09 indicates near-complete loss of endogenous insulin production from autoimmune beta-cell destruction, which is the pathophysiologic hallmark that distinguishes Type 1 diabetes from all other forms of diabetes. 1

Diagnostic Reasoning

C-Peptide as the Key Discriminator

• C-peptide is secreted in equimolar amounts with insulin and directly reflects pancreatic beta-cell function; a level this low essentially excludes Type 2 diabetes and MODY, both of which retain significant insulin production capacity. 1
• The American Diabetes Association specifically recommends using C-peptide levels to discriminate between Type 1 and Type 2 diabetes, as low C-peptide indicates the near-complete loss of endogenous insulin production characteristic of Type 1 diabetes. 1

Clinical Presentation Supports Type 1 Diabetes

• Polyuria and polydipsia are classic presenting symptoms of Type 1 diabetes in children, reflecting osmotic diuresis from hyperglycemia. 1
• The patient's age of 10 years falls within the peak incidence range for Type 1 diabetes onset in childhood. 1
• The HbA1c of 6.8% indicates chronic hyperglycemia over the preceding 2-3 months, consistent with progressive beta-cell destruction. 1

Why Other Diagnoses Are Excluded

Type 2 Diabetes (Option B) - Ruled Out

• Type 2 diabetes would show preserved or even elevated C-peptide levels due to insulin resistance with maintained beta-cell function, which is completely inconsistent with the profoundly low C-peptide of 0.09 seen in this patient. 1
• Type 2 diabetes in children typically presents with obesity, acanthosis nigricans, and family history of Type 2 diabetes—none of which override the definitive C-peptide evidence. 1

MODY (Option C) - Ruled Out

• MODY typically presents with mild hyperglycemia and preserved C-peptide levels, as these are genetic defects in beta-cell function rather than autoimmune destruction; the severely depleted C-peptide is pathophysiologically inconsistent with MODY. 1
• MODY patients maintain significant insulin secretory capacity despite their genetic beta-cell dysfunction. 1

Stress-Induced Hyperglycemia (Option D) - Ruled Out

• Stress-induced hyperglycemia is transient and would not produce an elevated HbA1c of 6.8%, which reflects months of sustained hyperglycemia. 1
• C-peptide would be normal or elevated in stress states due to counter-regulatory hormone responses, not profoundly suppressed to 0.09. 1

Critical Clinical Pitfalls to Avoid

• Never delay insulin initiation in a child with Type 1 diabetes presenting with polyuria and polydipsia, as these patients can rapidly progress to diabetic ketoacidosis. 2
• Do not be misled by a "borderline" HbA1c of 6.8%—in the context of profoundly low C-peptide and classic symptoms, this confirms the diagnosis regardless of the absolute HbA1c value. 1
• The C-peptide level is the definitive discriminator in this clinical scenario and overrides all other considerations when this profoundly suppressed. 1