Differential Diagnosis

The patient's presentation suggests a complex interplay of physical and psychological factors. Here's a categorized differential diagnosis:

• Single most likely diagnosis:

  • Anorexia Nervosa: The patient's low BMI (16 kg/m2), fatigue, and significant weight loss (inferred from the low BMI and the fact that she was a straight A student until recently, suggesting a decline in overall health and well-being) are highly suggestive of anorexia nervosa. The denial of feeling tired ("I haven't felt tired. I feel fine.") despite obvious physical signs of fatigue and the significant impact on her daily functioning (e.g., decline in academic performance) also aligns with the typical presentation of anorexia nervosa, where patients often deny or minimize their symptoms.

• Other Likely diagnoses:

  • Major Depressive Disorder (MDD) with significant somatic symptoms: Although the patient is already being treated for MDD, the severity of her physical symptoms (e.g., extreme fatigue, low blood pressure, bradycardia) could indicate a more severe depressive episode or inadequate treatment response.
  • Hypothyroidism: The patient's TSH level is slightly elevated (2.8 uU/mL), which could indicate hypothyroidism, a condition known to cause fatigue, weight loss, and dry skin. However, the clinical correlation and further testing would be necessary to confirm this diagnosis.

• Do Not Miss diagnoses:

  • Suicidal Ideation/Intent: Given the patient's history of major depressive disorder and current state of physical and emotional distress, it's crucial to assess for suicidal ideation or intent. The recent decline in academic performance and "relationship difficulties" could be contributing factors to an increased risk of suicide.
  • Infection or Sepsis: Although the patient's oxygen saturation is normal, and there's no immediate indication of infection, her low temperature (35.4°C or 95.7°F) and low blood pressure could suggest a systemic infection or sepsis, especially if not promptly recognized and treated.
  • Cardiac Issues (e.g., Bradycardia due to another cause): The patient's pulse is significantly low (37/min), which could be due to various cardiac issues, including but not limited to anorexia nervosa. Further cardiac evaluation might be necessary to rule out other causes of bradycardia.

• Rare diagnoses:

  • Addison's Disease: This is a rare endocrine disorder that could explain some of the patient's symptoms, such as fatigue, weight loss, and low blood pressure. However, specific diagnostic tests (e.g., cortisol levels) would be needed to consider this diagnosis further.
  • Mitochondrial Myopathies: These are a group of rare diseases that affect the mitochondria and could present with fatigue, among other symptoms. However, they would be much lower on the differential given the patient's age and the presence of more common explanations for her symptoms.

The physical examination finding most consistent with the underlying diagnosis (anorexia nervosa) would likely include signs of malnutrition and possibly A) Brittle hair and dry skin, which are common in individuals with significant malnutrition due to anorexia nervosa.