Differential Diagnosis

The patient's presentation with vomiting, severe diarrhea, fever, and a rash, along with a history of Addison's Disease and hypothyroidism, and recent exposure to Hand, Foot, and Mouth disease, suggests a complex clinical picture. Here's a categorized differential diagnosis:

• Single Most Likely Diagnosis
  • Viral gastroenteritis: Given the patient's symptoms of vomiting, severe diarrhea, and fever, along with the recent exposure to Hand, Foot, and Mouth disease, viral gastroenteritis is a plausible diagnosis. The rash described as more yeast-like could be a secondary infection or an unrelated skin condition.
• Other Likely Diagnoses
  • Adrenal crisis: The patient's history of Addison's Disease puts her at risk for an adrenal crisis, especially with symptoms like vomiting and diarrhea, which can lead to dehydration and electrolyte imbalances, exacerbating adrenal insufficiency.
  • Thyroid storm: Although less common, the patient's hypothyroidism history might suggest considering thyroid storm if she has untreated or undertreated hyperthyroidism, though this is less likely given her stated diagnosis of hypothyroidism.
  • Influenza or other viral respiratory infections: The patient's symptoms of fever and a stuffy nose could also suggest a viral respiratory infection, which might be complicated by her immunocompromised state due to chronic illnesses.
• Do Not Miss Diagnoses
  • Septic shock: Any patient presenting with fever, vomiting, and diarrhea could potentially be at risk for septic shock, especially if there's an underlying infection that's not immediately apparent. The patient's immunocompromised state increases this risk.
  • Dehydration and electrolyte imbalance: Given the symptoms of vomiting and diarrhea, dehydration and significant electrolyte imbalances are potential complications that could be life-threatening, especially in the context of Addison's Disease.
• Rare Diagnoses
  • Autoimmune enteropathy: A rare condition characterized by autoimmune destruction of the intestinal lining, which could present with severe diarrhea and might be considered in a patient with a history of autoimmune diseases like Addison's and hypothyroidism.
  • Opportunistic infections: Given the patient's immunocompromised state due to chronic illnesses, opportunistic infections (e.g., CMV, fungal infections) could be considered, especially if the patient is not responding to typical treatments for viral gastroenteritis or if there are other suggestive symptoms.

Each of these diagnoses should be considered in the context of the patient's overall clinical presentation, history, and laboratory findings. A thorough diagnostic workup, including laboratory tests and potentially imaging studies, would be necessary to determine the underlying cause of her symptoms.