Differential Diagnosis for a 16-year-old Girl with Rapid Weight Loss and Obsession over Her Weight

• Single most likely diagnosis:
  • Anorexia Nervosa: This is the most likely diagnosis given the patient's rapid weight loss, obsession over her weight, refusal to eat with the family, and lack of appetite. The patient's body mass index (BMI) of 17.8, which is below the normal range, further supports this diagnosis. Anorexia nervosa is a psychological eating disorder characterized by an intense fear of gaining weight and a distorted body image, leading to restricted food intake and significant weight loss.
• Other Likely diagnoses:
  • Bulimia Nervosa: Although the patient's symptoms primarily suggest anorexia nervosa, bulimia nervosa could also be considered, especially if the patient has episodes of binge eating followed by purging or other compensatory behaviors to prevent weight gain. However, the primary complaint of lack of appetite and refusal to eat suggests anorexia nervosa more strongly.
  • Other Specified Feeding or Eating Disorder (OSFED): This diagnosis applies to individuals who do not meet the full criteria for anorexia nervosa, bulimia nervosa, or other eating disorders but still have a significant eating disorder. Given the patient's symptoms, OSFED could be considered if she does not fully meet the criteria for anorexia nervosa.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.):
  • Hyperthyroidism: Although less likely, hyperthyroidism can cause significant weight loss, decreased appetite, and other symptoms that might mimic anorexia nervosa. It is crucial to rule out hyperthyroidism due to its potential for serious complications if left untreated.
  • Diabetes Mellitus: Uncontrolled diabetes can lead to weight loss and changes in appetite. Although the presentation might differ, it's essential to consider and rule out diabetes, especially if there are other suggestive symptoms like polyuria or polydipsia.
  • Malignancy: Rarely, significant weight loss in adolescents can be a sign of an underlying malignancy. While this is less likely, given the psychological and behavioral aspects of the patient's presentation, it is a diagnosis that must not be missed due to its severe implications.
• Rare diagnoses:
  • Brain Tumors: Certain brain tumors, especially those affecting the hypothalamus, can lead to changes in appetite and weight. This would be an extremely rare cause of the patient's symptoms but could be considered if other explanations are ruled out and there are neurological signs or symptoms.
  • Genetic Disorders: Some genetic disorders can affect appetite regulation and lead to significant weight loss. Examples include Prader-Willi syndrome, though the age of onset and other associated features would typically make this less likely in a 16-year-old presenting with new symptoms.