A 17-year-old girl is admitted to the hospital for medical stabilization of severe anorexi… — USMLE Step 2 CK

A 17-year-old girl is admitted to the hospital for medical stabilization of severe anorexia nervosa, restricting subtype. Her body mass index on admission is $12.5 \text{ kg/m}^2$ . Her home medication of fluoxetine is continued. Routine admission laboratories reveal a normal thyroid-stimulating hormone (TSH) level with a low total T3 and low-normal free T4. Due to her profound malnutrition, continuous nasogastric tube feeding is initiated at a high caloric rate to promote rapid weight gain. On the third day of hospitalization, the patient suddenly develops significant shortness of breath, a cough productive of pink frothy sputum, and orthopnea. Her temperature is $37.1^\circ\text{C}$ ( $98.8^\circ\text{F}$ ), pulse is $118\text{/min}$ , respirations are $28\text{/min}$ , and blood pressure is $92/60\text{ mmHg}$ . Physical examination reveals distinct jugular venous distention and bilateral basilar lung crackles. Which of the following is the most likely primary mechanism for this patient's acute clinical deterioration?

Intracellular shift of phosphate resulting in myocardial adenosine triphosphate depletionAnswer
B
Acute airway inflammation, smooth muscle constriction, and mucous plugging
C
Inadequate respiratory compensation for a severe accumulation of serum ketoacids
D
Diminished myocardial contractility and pericardial effusion due to primary hypothyroidism
E
Synergistic hyperactivation of central and peripheral serotonin receptors

Answer

The correct mechanism is an intracellular shift of phosphate resulting in myocardial adenosine triphosphate (ATP) depletion.

This patient is experiencing refeeding syndrome, a classic and highly lethal complication of rapid nutritional rehabilitation in profoundly malnourished individuals (such as those with severe anorexia nervosa). Introducing a high-carbohydrate load triggers a massive surge in insulin secretion. Insulin drives the cellular uptake of glucose along with phosphate, potassium, and magnesium. Because these patients already have severely depleted total body stores of these minerals (even if initial serum levels appear normal), this sudden intracellular shift causes profound hypophosphatemia, hypokalemia, and hypomagnesemia. Phosphate is fundamentally essential for the production of adenosine triphosphate (ATP) and 2,3-diphosphoglycerate (2,3-DPG). The rapid depletion of ATP results in generalized cellular dysfunction, notably myocardial contractility failure (acute congestive heart failure), while low 2,3-DPG impairs oxygen release to tissues. The patient's acute jugular venous distention, orthopnea, pink frothy sputum, and crackles are direct signs of this cardiogenic pulmonary edema.

Step-by-Step Solution

Analyze the patient's acute symptoms and physical examination findings.

The combination of significant shortness of breath, orthopnea, pink frothy sputum, tachycardia, hypotension, jugular venous distention, and basilar crackles indicates acute decompensated heart failure and pulmonary edema.

Accurately identifying the clinical syndrome (cardiogenic failure vs. primary pulmonary disease) is the essential first step in determining the underlying mechanism.

Correlate the acute heart failure with the patient's specific clinical context.

The patient is profoundly malnourished (BMI 12.5) and recently started on a high-calorie enteral feeding regimen. This specific timeline and trigger point strongly to refeeding syndrome.

Medical stabilization of severe eating disorders requires careful management to prevent complications of nutritional rehabilitation.

Determine the biochemical mechanism of refeeding syndrome that leads to heart failure.

A high carbohydrate load triggers a massive insulin surge, which drives phosphate, magnesium, and potassium into cells. The resulting severe hypophosphatemia leads to depleted ATP and 2,3-DPG, causing myocardial tissue hypoxia and contractility failure.

Understanding the precise electrolyte shift distinguishes refeeding syndrome from other causes of deterioration like endocrine abnormalities or drug interactions.

Key Concept

Refeeding syndrome is a life-threatening complication of rapid nutritional rehabilitation in severe anorexia nervosa, characterized by insulin-driven intracellular shifts of phosphate, potassium, and magnesium, leading to ATP depletion and subsequent cardiopulmonary failure.

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