A 22-year-old female college student presents to the psychiatry clinic with a 3-year history of restrictive eating and intense fear of weight gain. She reports consuming only 800–1000 kcal daily, exercising excessively for 2–3 hours, and weighing herself multiple times per day. On examination, BMI is 16.2 kg/m², heart rate 52 bpm, and blood pressure 98/62 mmHg. Laboratory investigations reveal serum potassium 3.1 mEq/L, magnesium 1.6 mg/dL, and phosphate 2.0 mg/dL. ECG shows prolonged QTc interval. What is the most likely diagnosis?

Explanation

## Diagnosis: Anorexia Nervosa, Restricting Type ### Clinical Presentation This patient meets DSM-5 criteria for anorexia nervosa (AN): - **Restriction of energy intake** leading to significantly low body weight (BMI 16.2, below expected minimum) - **Intense fear of weight gain** despite being underweight - **Disturbance in body image** (implied by excessive exercise and frequent weighing) - **Restricting type**: weight loss achieved through dieting/exercise alone, without regular binge eating or purging episodes ### Key Clinical Features **Key Point:** Anorexia nervosa is characterized by severe caloric restriction and a relentless pursuit of thinness, with medical complications arising from malnutrition and electrolyte derangement. ### Medical Complications | Complication | Finding in This Case | Mechanism | |---|---|---| | **Hypokalemia** | K⁺ 3.1 mEq/L (normal 3.5–5.0) | Renal potassium wasting, poor intake | | **Hypomagnesemia** | Mg 1.6 mg/dL (normal 1.7–2.2) | Gastrointestinal malabsorption | | **Hypophosphatemia** | PO₄ 2.0 mg/dL (normal 2.5–4.5) | Malnutrition, renal losses | | **Bradycardia** | HR 52 bpm | Adaptive response to caloric deficit; risk for arrhythmia | | **Prolonged QTc** | On ECG | Electrolyte abnormalities (K⁺, Mg²⁺, Ca²⁺) increase sudden cardiac death risk | **Clinical Pearl:** The triad of hypokalemia, hypomagnesemia, and prolonged QTc in a malnourished patient is a medical emergency—refeeding syndrome and sudden cardiac death are imminent risks. **High-Yield:** Anorexia nervosa has the highest mortality rate of any psychiatric disorder (~5–10% over 10 years), primarily due to cardiac arrhythmias and suicide. ### Distinguishing Restricting vs. Purging Type | Feature | Restricting Type | Purging Type | |---|---|---| | **Weight loss mechanism** | Diet, exercise, fasting | Binge eating + self-induced vomiting, laxatives, diuretics | | **Binge eating** | Absent or rare | Regular episodes | | **Electrolyte pattern** | Hypokalemia (renal losses) | Hypokalemia + metabolic alkalosis (from vomiting) | | **Dental/GI signs** | Minimal | Dental erosion, parotid enlargement, esophageal tears | This patient has **no history of binge eating or purging**, making restricting type the correct diagnosis. **Mnemonic: AN-FEAR** — Anorexia Nervosa: **F**ear of weight gain, **E**nergy restriction, **A**bnormally low weight, **R**elentless pursuit of thinness. ### Management Priorities 1. **Immediate:** Correct severe electrolyte abnormalities (especially K⁺, Mg²⁺, PO₄) to prevent arrhythmias 2. **Nutritional rehabilitation:** Gradual refeeding (risk of refeeding syndrome with rapid caloric increase) 3. **Psychotherapy:** Cognitive-behavioral therapy (CBT) or family-based therapy (FBT) 4. **Medical monitoring:** Cardiac surveillance, bone density assessment (osteoporosis risk) [cite:DSM-5 Feeding and Eating Disorders], [cite:Harrison 21e Ch 380]