What is the most likely cause of the current condition in an 18‑year‑old presenting with vomiting, diarrhea, decreased oral intake, lethargy, fruity (acetone) breath, mild hypotension, random blood glucose 8.9 mmol/L, serum sodium 124 mmol/L, and bicarbonate 15 mmol/L?

Diabetic Ketoacidosis (DKA)

The most likely diagnosis is Diabetic Ketoacidosis (DKA), despite the relatively modest blood glucose of 8.9 mmol/L (160 mg/dL), based on the classic triad of fruity breath (acetone), metabolic acidosis (bicarbonate 15 mmol/L), and clinical presentation with vomiting and lethargy.

Clinical Reasoning

This 18-year-old presents with the pathognomonic features of DKA:

• Fruity (acetone) breath - This is the hallmark clinical sign of ketosis and essentially confirms significant ketone body production 1
• Metabolic acidosis - Bicarbonate of 15 mmol/L meets DKA diagnostic criteria (bicarbonate <18 mEq/L for mild DKA) 1
• Appropriate clinical context - Vomiting, diarrhea, and decreased oral intake are classic precipitating factors and presenting symptoms 1
• Lethargy and mild hypotension - Consistent with dehydration and metabolic derangement seen in DKA 1

Critical Diagnostic Pitfall: Euglycemic DKA

The blood glucose of 8.9 mmol/L (160 mg/dL) is below the typical DKA threshold of 250 mg/dL, but this does NOT exclude DKA. This represents "euglycemic" or "pseudonormoglycemic" DKA, which can occur in several scenarios 2, 3:

• Decreased oral intake and vomiting (as in this patient) limiting glucose availability
• Partial insulin effect or recent insulin administration
• SGLT2 inhibitor use (though not mentioned here)
• Severe hypertriglyceridemia causing laboratory interference 3

The presence of fruity breath is far more specific for ketoacidosis than the glucose level alone and should override concerns about the "low" glucose 4.

Why Not the Other Options?

Option B (Uncontrolled Hyperglycemia): This is too vague and doesn't explain the acidosis, fruity breath, or clinical severity.

Option C (Hyponatremia): While the sodium is 124 mmol/L, this is likely pseudohyponatremia from hyperglycemia (each 100 mg/dL glucose elevation decreases sodium by ~1.6 mEq/L) or dilutional from fluid shifts. Hyponatremia alone doesn't cause fruity breath or metabolic acidosis 1.

Option D (Hyperosmolar Hyperglycemic State): HHS requires glucose ≥600 mg/dL and typically presents with more profound altered mental status without significant ketosis 1. The fruity breath and acidosis exclude this diagnosis.

Immediate Management Algorithm

Based on established guidelines 1:

1. Confirm diagnosis immediately:

   • Obtain venous blood gas (pH expected <7.3)
   • Check serum or urine ketones (will be strongly positive)
   • Calculate anion gap (likely elevated >12)

2. Initiate fluid resuscitation:

   • 0.9% NaCl at 15-20 mL/kg/h in first hour (1-1.5 L for average adult) 1

3. Start insulin therapy:

   • For mild DKA: 0.4-0.6 units/kg priming dose (half IV, half SC/IM), then 0.1 unit/kg/h SC or IM 1
   • For moderate-severe: continuous IV insulin infusion preferred 1

4. Monitor closely:

   • Blood glucose, electrolytes, venous pH every 2-4 hours 1
   • Watch for hypokalemia during treatment
   • Gradual correction to avoid cerebral edema (especially critical in young patients) 1

5. Resolution criteria:

   • Glucose <200 mg/dL
   • Bicarbonate ≥18 mEq/L
   • Venous pH >7.3 1

Key Clinical Pearls

• Never dismiss DKA based on "normal" glucose alone - euglycemic DKA is increasingly recognized and can be life-threatening 2, 3
• Fruity breath is diagnostic - if present with acidosis, treat as DKA regardless of glucose level 4
• The hyponatremia will likely correct with treatment of the hyperglycemic crisis and should not distract from the primary diagnosis 1
• Persistent acidosis despite treatment should prompt evaluation for alternative causes (lactic acidosis, bowel ischemia, etc.) 5