Welcome to USMLE practice question #151! Today’s topic is Endocrinology.
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A 42-year-old woman with a history of anorexia nervosa is admitted to the hospital for severe malnutrition. She reports consuming minimal food for the past 3 weeks. Her initial body mass index is 15 kg/m². Laboratory results before initiating nutritional therapy are as follows:
• Sodium: 137 mEq/L
• Potassium: 3.4 mEq/L
• Phosphorus: 2.8 mg/dL
• Magnesium: 1.5 mg/dL
The patient is started on a high-calorie enteral feeding regimen. Two days later, she develops generalized weakness, confusion, and peripheral edema. Repeat laboratory results reveal:
• Sodium: 138 mEq/L
• Potassium: 2.8 mEq/L
• Phosphorus: 1.1 mg/dL
• Magnesium: 1.0 mg/dL
Which of the following is the primary mechanism responsible for this patient’s condition?
A) Increased insulin secretion leading to intracellular shift of electrolytes
B) Increased renal excretion of electrolytes due to fluid overload
C) Impaired gastrointestinal absorption of electrolytes due to malnutrition
D) Reduced hepatic production of albumin leading to fluid shifts
E) Systemic inflammation causing cellular electrolyte depletion
Detailed Breakdown of Answers + Correct Answer Below ⏬
GENERAL ANALYSIS
A 42-year-old woman with a history of anorexia nervosa and severe malnutrition presents with generalized weakness, confusion, and peripheral edema after starting high-calorie enteral feeding.
ANSWER CHOICES:
CHOICE A: Increased insulin secretion leading to intracellular shift of electrolytes
Explanation: Insulin promotes cellular uptake of glucose along with phosphorus, potassium, and magnesium, leading to a rapid depletion of these electrolytes in the serum. Phosphorus is particularly critical for ATP production; its depletion leads to cellular dysfunction, weakness, and confusion. Fluid retention occurs due to sodium and water shifts, contributing to peripheral edema.
CHOICE B: Increased renal excretion of electrolytes due to fluid overload
Explanation: Fluid overload can cause renal losses of electrolytes due to pressure natriuresis or diuresis. However, this patient’s electrolyte abnormalities suggest an underlying cause different from renal losses. The hallmark hypophosphatemia seen here is driven by intracellular shifts, not renal excretion.
CHOICE C: Impaired gastrointestinal absorption of electrolytes due to malnutrition
Explanation: Chronic malnutrition can impair gastrointestinal absorption of some nutrients. However, this would lead to chronically low electrolyte levels rather than the acute drop seen after initiating feeding. The rapid electrolyte changes after feeding are better explained by intracellular shifts rather than impaired absorption.
CHOICE D: Reduced hepatic production of albumin leading to fluid shifts
Explanation: Hypoalbuminemia can cause fluid shifts and edema by reducing oncotic pressure. However, this does not explain the acute electrolyte abnormalities or symptoms such as weakness and confusion. Peripheral edema in this patient’s specific scenario is caused by sodium and water retention rather than hypoalbuminemia.
CHOICE E: Systemic inflammation causing cellular electrolyte depletion
Explanation: Systemic inflammation can lead to electrolyte disturbances in conditions like sepsis or critical illness. However, this patient’s presentation is unlikely caused by an inflammatory process—there is no evidence of systemic inflammation in this patient.
FINAL VERDICT…
CORRECT ANSWER: A) Increased insulin secretion leading to intracellular shift of electrolytes
The primary mechanism of refeeding syndrome is increased insulin secretion, which drives an intracellular shift of electrolytes, particularly phosphorus, potassium, and magnesium.
KEY CONCEPTS:
Refeeding Syndrome Overview:
Triggered by rapid reintroduction of nutrition in malnourished patients.
Hallmark feature: Hypophosphatemia due to increased cellular uptake driven by insulin.
Other findings: Hypokalemia, hypomagnesemia, fluid retention.
Electrolyte Changes in Refeeding Syndrome:
Phosphorus (↓): Cellular uptake for ATP production.
Potassium (↓): Cellular uptake via insulin stimulation.
Magnesium (↓): Cellular uptake and utilization in metabolic processes.
Management of Refeeding Syndrome:
Gradual reintroduction of nutrition (start with low-calorie feeds).
Close monitoring and correction of electrolytes (phosphorus, potassium, magnesium).
Avoid overhydration to prevent fluid overload.