A 16-year-old girl presents with appetite, insomnia, and amenorrhea for 3 months. What is the most likely diagnosis, and how will you confirm it?

The most likely diagnosis for a 16-year-old girl presenting with appetite changes, insomnia, and amenorrhea for 3 months is an eating disorder, specifically anorexia nervosa.

To confirm the diagnosis, a thorough physical examination and medical history should be conducted. Additionally, blood tests may be ordered to check for any abnormalities in electrolyte levels, thyroid function, and hormone levels. A psychological evaluation may also be necessary to assess the patient's mental health and emotional well-being. In some cases, imaging studies such as bone density scans may be recommended to assess the impact of malnutrition on the patient's bone health. A multidisciplinary approach involving a team of healthcare professionals, including a psychiatrist, psychologist, and dietitian, may be necessary for a comprehensive evaluation and treatment plan.

Most likely diagnosis: Anorexia nervosa (AN)

Confirmation:
1. History and Physical Examination:
- Obtain a detailed medical and psychiatric history, including food intake, weight loss, and menstrual irregularities.
- Conduct a physical examination, including measurements of vital signs, weight, body mass index (BMI), and physical symptoms such as cachexia, lanugo hair, and bradycardia.

2. Diagnostic Criteria:

- Restriction of energy intake leading to significantly low body weight (BMI <17.5 for adults or BMI <5th percentile for children/adolescents)
- Intense fear of gaining weight or becoming fat
- Disturbance in the way body weight or shape is experienced, undue influence of weight on self-evaluation, or denial of the seriousness of the current low weight
- Absence of menstruation for at least 3 months in females

3. Laboratory Tests:
- Complete blood count: Anemia, leukopenia, and thrombocytopenia
- Electrolytes: Hyponatremia, hypokalemia, and hypochloremia
- Liver function tests: Elevated alanine aminotransferase (ALT) and aspartate aminotransferase (AST)
- Thyroid function tests: May show elevated thyroid-stimulating hormone (TSH) due to hypothalamic-pituitary-adrenal (HPA) axis dysregulation

4. Imaging:
- Dual-energy X-ray absorptiometry (DEXA) scan: Low bone mineral density

5. Psychological Evaluation:
- Assessment of body image, self-esteem, and coping mechanisms
- Exploration of underlying psychological factors contributing to the eating disorder

Differential Diagnoses:
- Restrictive food intake disorder
- Depressive disorder with prominent anhedonia
- Substance use disorder
- Sleep disorder
- Endocrine disorders (e.g., hypothyroidism)

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