Anorexia Nervosa

USMLE Step 1 trap: Applies the outdated DSM-IV amenorrhea criterion to anorexia nervosa diagnosis. DSM-5 removed amenorrhea as a required criterion, so anorexia can be diagnosed in males, premenarchal females, and women using hormonal contraception.

Anorexia nervosa is one of the highest-yield eating disorder topics on USMLE Step 1, and it's tested in more ways than most students expect. The core is a triad: restriction of energy intake leading to significantly low body weight, intense fear of gaining weight, and disturbed body image. There are two subtypes — restricting type (pure caloric restriction and exercise) and binge-purge type (restriction plus episodic binge eating or purging). The exam will ask you to identify the diagnosis, distinguish subtypes, and recognize that anorexia carries the highest mortality of any psychiatric disorder.

Step 1 tests anorexia from four angles: diagnostic criteria, medical complications, refeeding syndrome physiology, and management. Criteria questions often hinge on what DSM-5 changed from DSM-IV. Complication questions present a clinical scenario with physical findings — lanugo, bradycardia, hypotension, electrolyte abnormalities — and ask you to connect them to the pathophysiology. Refeeding syndrome questions are pure mechanism: why does a specific electrolyte shift in a specific direction when you restart nutrition? Management questions probe what actually works versus what students assume works.

The trickiest part of this topic is that multiple classic 'facts' are outdated or just wrong. Students still think amenorrhea is required for diagnosis (it was removed in DSM-5). Students reach for SSRIs as first-line treatment (there's no proven pharmacotherapy for AN). And refeeding syndrome trips up almost everyone — the instinct is that giving nutrition causes high phosphate, but the opposite is true. Lock down those misconceptions early and this topic becomes very manageable.

From real student decks

68%have cards covering this topic

57%have mature cards

Common misconceptions

Common mistake

Wrong: Amenorrhea is still a required criterion for diagnosing anorexia nervosa.

Right: DSM-5 removed amenorrhea as a required criterion, so anorexia can be diagnosed in males, premenarchal females, and women using hormonal contraception.

DSM-IV listed amenorrhea as a required fourth criterion for anorexia nervosa, but DSM-5 explicitly removed it. This change was made because the criterion excluded males, premenarchal girls, and women on hormonal contraception who otherwise met all criteria. On USMLE Step 1, if a vignette presents a male or a patient on oral contraceptives with classic anorexia features, the diagnosis is still anorexia nervosa — don't let the absence of amenorrhea talk you out of it.

Common mistake

Wrong: Refeeding syndrome causes hyperphosphatemia because the patient is receiving nutrition.

Right: Refeeding syndrome causes hypophosphatemia (along with hypokalemia and hypomagnesemia) because insulin release drives phosphate into cells when carbohydrates are reintroduced.

The intuition that 'giving nutrition raises phosphate' is backwards. In refeeding syndrome, carbohydrate reintroduction triggers insulin release, which drives glucose — and phosphate — into cells. The patient was already phosphate-depleted from starvation, so serum phosphate crashes dramatically. The same insulin surge also causes hypokalemia and hypomagnesemia. The danger is cardiac arrhythmias and respiratory failure from severe hypophosphatemia, which is why you monitor electrolytes closely and correct them before and during refeeding.

Common mistake

Wrong: SSRIs are first-line pharmacotherapy for anorexia nervosa.

Right: No pharmacotherapy has proven efficacy for anorexia nervosa; nutritional rehabilitation and psychotherapy (CBT, family-based therapy) are the mainstays, and SSRIs are not recommended as primary treatment.

SSRIs are effective for bulimia nervosa, so students naturally reach for them in anorexia nervosa too — but the evidence simply isn't there. Multiple trials have failed to show that SSRIs improve weight restoration or core anorexia symptoms. The reason is partly that serotonin function depends on adequate tryptophan and caloric intake, so the drug can't work properly in a severely malnourished brain. The mainstay of treatment is nutritional rehabilitation first, then psychotherapy (CBT in adults, family-based therapy in adolescents); pharmacotherapy has no established primary role.

Common mistake

Gap: Missing the specific medical thresholds that mandate inpatient admission in anorexia nervosa

Inpatient admission for anorexia is triggered by weight <85% of ideal body weight, heart rate <50 bpm, QTc prolongation, syncope, severe electrolyte disturbances, or failure of outpatient treatment.

Outpatient management fails when medically dangerous thresholds are crossed, and the exam expects you to know these numbers. Inpatient admission is indicated for weight below 85% of ideal body weight, heart rate below 50 bpm, QTc prolongation, syncope, severe electrolyte disturbances (especially hypokalemia or hypophosphatemia), or failure of outpatient treatment. Bradycardia is the most commonly tested trigger — it reflects vagal tone dominance from starvation-induced cardiac remodeling and signals imminent risk of fatal arrhythmia.

Free Deck audit

See if your Anki deck covers this topic.

Upload your deck →

Guided session

Stuck on this? An AI tutor that probes your understanding.

Start a session →

What the exam tests

Know the four DSM-5 criteria for anorexia nervosa and be able to distinguish the restricting subtype (no binge-purge behavior) from the binge-purge subtype (episodes of bingeing or purging despite overall restriction).
Recognize the medical complications of anorexia — including lanugo, bradycardia, orthostatic hypotension, osteoporosis, and electrolyte abnormalities — and identify which findings mandate inpatient admission rather than outpatient management.
Explain the mechanism of refeeding syndrome: why reintroducing carbohydrates causes hypophosphatemia (plus hypokalemia and hypomagnesemia), and what the prevention strategy is.
Identify that nutritional rehabilitation and psychotherapy (CBT for adults, family-based therapy for adolescents) are the mainstays of anorexia treatment, and that no pharmacotherapy has established first-line efficacy.

Can you avoid these mistakes?

A 17-year-old male presents with BMI of 15.5, intense fear of gaining weight, and distorted body image. He restricts calories severely but denies any binge eating or purging. His girlfriend mentions he runs 10 miles daily. What is the diagnosis and subtype, and what criterion would have prevented this diagnosis under DSM-IV?

A 24-year-old woman with anorexia nervosa (BMI 14) is admitted for nutritional rehabilitation. On day 3 of refeeding, she develops muscle weakness and her EKG shows new dysrhythmia. Which electrolyte abnormality is most responsible, what is the mechanism, and what other electrolytes are also at risk?

A patient with anorexia nervosa is medically stable with a BMI of 16.5 and no acute electrolyte abnormalities. Her psychiatrist considers starting fluoxetine. Is this appropriate, and what does the evidence say about pharmacotherapy for anorexia nervosa compared to bulimia nervosa?

You are evaluating a 19-year-old woman with known anorexia nervosa in an outpatient clinic. Her weight is 83% of ideal body weight, resting heart rate is 46 bpm, and she denies syncope. Which of these findings alone is sufficient to trigger inpatient admission, and what are the other major thresholds you should know?

Anorexia Nervosa

Common misconceptions

What the exam tests

Can you avoid these mistakes?

Related topics