8M + recent viral illness + severe lethargy + polyuria + polydipsia
This classic presentation is highly suggestive of Diabetic Ketoacidosis (DKA) as the initial presentation of Type 1 Diabetes Mellitus (T1DM), often precipitated by a viral illness.
Here is the breakdown of the expected findings:
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Serum Na+: ↓
- Reasoning: Severe hyperglycemia acts as an osmotic agent, pulling water from the intracellular space into the extracellular space. This leads to a dilutional hyponatremia. Despite this, the patient is severely dehydrated due to osmotic diuresis (water loss > Na+ loss). High-yield point: Always calculate the corrected sodium in DKA.
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Serum K+: ↑ or Normal
- Reasoning: Despite a total body potassium deficit, serum K+ is often elevated or normal. This is due to two key mechanisms:
- Lack of Insulin: Insulin normally drives K+ into cells. Without it, K+ remains in the serum.
- Acidosis: Excess H+ in the blood is buffered by shifting into cells in exchange for K+, which shifts out.
- Reasoning: Despite a total body potassium deficit, serum K+ is often elevated or normal. This is due to two key mechanisms:
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Total Body K+: ↓
- Reasoning: Significant K+ is lost in the urine due to osmotic diuresis and vomiting. This leads to a profound total body potassium deficit, which is unmasked when treatment with insulin is initiated. High-yield point: This is why K+ replacement is critical during DKA treatment, even if the initial serum K+ is high.
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Glomerular Filtration Rate (GFR): ↓
- Reasoning: The massive fluid loss from osmotic diuresis leads to hypovolemia and decreased renal perfusion. This causes a prerenal acute kidney injury, resulting in a decreased GFR.