# Situation: A 52-year-old man with alcohol use disorder is admitted for poor oral intake. He has eaten very little for the past 12 days. He is 1.70 m tall, weighs 48 kg today, and his usual weight is 60 kg. The team plans nasogastric (NG) tube feeding. He has no documented heart, kidney, or liver failure. On the second day of feeding he reports weakness and mild shortness of breath. His results are (normal ranges in parentheses): potassium 3.3 mEq/L (3.5–5.0 mEq/L); phosphate 0.9 mg/dL (2.5–4.5 mg/dL); magnesium 1.5 mg/dL (1.7–2.2 mg/dL); sodium 137 mEq/L (135–145 mEq/L); capillary glucose 142 mg/dL (7.9 mmol/L). Which finding BEST explains his new symptoms?

> source: MyMerci (mymerci.kr)  
> url: https://mymerci.kr/pages/nclex_q.php?qn_id=630269  
> language: ko  
> subject: Nursing Practice IV — Care of Clients with Problems in Nutrition and Gastrointestinal, Metabolism and Endocrine, Perception and Coordination

## 문제

Situation: A 52-year-old man with alcohol use disorder is admitted for poor oral intake. He has eaten very little for the past 12 days. He is 1.70 m tall, weighs 48 kg today, and his usual weight is 60 kg. The team plans nasogastric (NG) tube feeding. He has no documented heart, kidney, or liver failure.

On the second day of feeding he reports weakness and mild shortness of breath. His results are (normal ranges in parentheses): potassium 3.3 mEq/L (3.5–5.0 mEq/L); phosphate 0.9 mg/dL (2.5–4.5 mg/dL); magnesium 1.5 mg/dL (1.7–2.2 mg/dL); sodium 137 mEq/L (135–145 mEq/L); capillary glucose 142 mg/dL (7.9 mmol/L). Which finding BEST explains his new symptoms?

## 보기

1. Low potassium weakening skeletal and breathing muscles
2. Low magnesium causing neuromuscular irritability
3. Rising glucose causing osmotic fluid loss and fatigue
4. Low phosphate depleting energy for breathing muscles **✔ 정답**

**정답: 4**

## 해설

Refeeding lowers phosphate, potassium, and magnesium, but the phosphate here is the most severely reduced (well under half the lower limit). Severe hypophosphatemia depletes cellular energy (ATP) and oxygen delivery, causing muscle weakness and respiratory muscle failure, so it is the finding that explains weakness with shortness of breath. The feeding plan is reviewed and phosphate is replaced per protocol.

## 심화 해설

Core mechanism of refeeding syndrome

When a severely malnourished patient resumes nutrition, insulin secretion rises in response to carbohydrate intake. Insulin drives phosphate, potassium, and magnesium from the extracellular space into cells, and phosphate is also consumed for ATP synthesis and glycolysis. The result is a rapid drop in serum electrolytes. Among the three classic electrolyte shifts, phosphate falls most dramatically and is the defining abnormality of refeeding syndrome. In this patient, phosphate is 0.9 mg/dL against a normal range of 2.5–4.5 mg/dL, meaning it has dropped to roughly one-third of the lower limit. Potassium is mildly low at 3.3 mEq/L and magnesium is borderline low at 1.5 mg/dL, but neither is as severely depleted as phosphate.

Why severe hypophosphatemia explains weakness and dyspnea

Phosphate is required to synthesize ATP, to maintain the 2,3-diphosphoglycerate (2,3-DPG) content of red blood cells, and to support cell membrane integrity. When serum phosphate falls below roughly 1.0 mg/dL, ATP production in skeletal muscle becomes inadequate. Respiratory muscles, especially the diaphragm, are highly dependent on continuous ATP supply, so severe hypophosphatemia produces disproportionate respiratory muscle weakness. In addition, low 2,3-DPG shifts the oxyhemoglobin dissociation curve leftward, reducing oxygen release to tissues and worsening tissue hypoxia. The combination of impaired energy production and reduced oxygen delivery explains why this patient reports both generalized weakness and mild shortness of breath on the second day of feeding.

Comparing the electrolyte abnormalities

| Finding | Patient value | Normal range | Severity | Clinical link to symptoms |
| --- | --- | --- | --- | --- |
| Phosphate | 0.9 mg/dL | 2.5–4.5 mg/dL | Severe (below 1.0 mg/dL) | ATP depletion, diaphragm weakness, reduced oxygen delivery |
| Potassium | 3.3 mEq/L | 3.5–5.0 mEq/L | Mild | Can cause weakness, but not typically respiratory failure at this level |
| Magnesium | 1.5 mg/dL | 1.7–2.2 mg/dL | Mild | Neuromuscular irritability, but less directly tied to dyspnea here |
| Glucose | 142 mg/dL | Fasting

## 임상 시나리오

Refeeding Syndrome: Phosphate FirstRecognizing severe hypophosphatemia in the refed malnourished patient
When nutrition resumes in a severely malnourished patient, insulin drives phosphate, potassium, and magnesium into cells. Phosphate falls most dramatically and is the defining abnormality of refeeding syndrome.

A serum phosphate below 1.0 mg/dL is severe. Phosphate is required for ATP synthesis and 2,3-DPG production. Low ATP causes respiratory muscle weakness, especially of the diaphragm, producing dyspnea. Low 2,3-DPG shifts the oxyhemoglobin curve leftward, reducing oxygen delivery to tissues.

In this patient, phosphate is 0.9 mg/dL (normal 2.5–4.5 mg/dL), roughly one-third of the lower limit. Potassium (3.3 mEq/L) and magnesium (1.5 mg/dL) are only mildly reduced and do not explain the acute weakness and shortness of breath.

CautionDo not attribute new weakness and dyspnea in a refed patient to mild hypokalemia or hypomagnesemia alone. Check phosphate immediately; severe hypophosphatemia requires phosphate replacement and review of the feeding plan before respiratory failure develops.

## 핵심 개념

- **Refeeding syndrome** — Metabolic and electrolyte shifts occurring when nutrition is reintroduced to a severely malnourished patient, characterized by hypophosphatemia, hypokalemia, and hypomagnesemia
- **Hypophosphatemia** — Low serum phosphate level; severe hypophosphatemia below 1.0 mg/dL impairs ATP synthesis and causes muscle weakness including respiratory failure
- **2,3-DPG** — 2,3-diphosphoglycerate in red blood cells that facilitates oxygen release from hemoglobin; low phosphate reduces 2,3-DPG, shifting the oxyhemoglobin curve leftward
- **ATP** — Adenosine triphosphate, the primary energy currency of cells; phosphate is essential for its synthesis
- **NG tube feeding** — Nasogastric tube feeding used to deliver nutrition enterally in patients who cannot eat adequately by mouth

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