Differential Diagnosis

The patient's presentation includes a rash, reddish burns on the chin, painful stools, crampy abdominal pain, weight loss, and erythematous tender nodules on the lower extremities. Considering these symptoms, the differential diagnosis can be categorized as follows:

• Single Most Likely Diagnosis

  • Inflammatory Bowel Disease (IBD): The combination of abdominal pain, loose stools, weight loss, and the presence of erythematous tender nodules (which could be consistent with erythema nodosum, a known extraintestinal manifestation of IBD) makes IBD a strong candidate. The symptoms of IBD can include chronic diarrhea, abdominal pain, weight loss, and extraintestinal manifestations such as skin and joint involvement.

• Other Likely Diagnoses

  • IgA Vasculitis: This condition, formerly known as Henoch-Schönlein purpura, can present with abdominal pain, gastrointestinal bleeding, and a characteristic purpuric rash. While it more commonly affects children, it can occur in adults. The presence of a rash and abdominal symptoms could align with IgA vasculitis, although the age and some symptoms might not perfectly fit the typical profile.
  • Celiac Disease: Although celiac disease primarily causes gastrointestinal symptoms like diarrhea and abdominal pain due to gluten sensitivity, it can also have extraintestinal manifestations, including skin rashes (like dermatitis herpetiformis). However, the presence of erythematous nodules and the specific pattern of weight loss and abdominal pain might not as closely align with celiac disease as with IBD.

• Do Not Miss Diagnoses

  • Colon Cancer: Although less likely in a 20-year-old due to the lower incidence of colon cancer in young adults, unexplained weight loss, changes in bowel habits, and abdominal pain could be indicative of a malignancy. It's crucial to rule out colon cancer due to its potential severity and the importance of early detection.
  • Infectious Causes: Certain infections could mimic the symptoms described, including bacterial, viral, or parasitic infections affecting the gastrointestinal tract. These could potentially cause similar symptoms and should be considered, especially if the patient has been exposed to risky foods, water, or has traveled recently.

• Rare Diagnoses

  • Behçet's Disease: This rare condition can cause a wide range of symptoms, including oral and genital ulcers, skin lesions, and gastrointestinal symptoms. While it's less likely, the combination of skin and gastrointestinal symptoms could potentially fit a diagnosis of Behçet's disease.
  • Sarcoidosis: This autoimmune disease can affect multiple parts of the body, including the skin, lungs, and gastrointestinal tract. It could potentially explain some of the patient's symptoms, although it would be considered a less common diagnosis given the specific constellation of symptoms presented.