Differential Diagnosis for a 44-year-old Male with Diarrhea, Fever, Dysuria, and Rash

Single Most Likely Diagnosis

• Reactive Arthritis (Reiter's Syndrome): This condition is characterized by the development of arthritis, urethritis, and conjunctivitis following a gastrointestinal or genitourinary infection. The patient's symptoms of diarrhea, fever, dysuria, and rash, along with the urinalysis findings, make this a plausible diagnosis. The presence of leukocytes and nitrites in the urine suggests a urinary tract infection, which could be a trigger for reactive arthritis.

Other Likely Diagnoses

• Urinary Tract Infection (UTI) with Systemic Symptoms: The patient's dysuria, positive urinalysis (leukocytes and nitrites), and proteinuria are consistent with a UTI. The fever and rash could be systemic manifestations of the infection.
• Gastroenteritis with Extraintestinal Manifestations: Although the gastrointestinal symptoms have improved, some pathogens (e.g., Salmonella, Shigella) can cause extraintestinal manifestations, including urinary symptoms and rash.
• Sexually Transmitted Infection (STI): The patient's dysuria and urinalysis results could also suggest an STI, such as gonorrhea or chlamydia, which can cause systemic symptoms, including fever and rash.

Do Not Miss Diagnoses

• Septicemia or Bacteremia: Although the patient appears stable, the combination of fever, diarrhea, and urinary symptoms could indicate a systemic infection. It is crucial to consider and rule out septicemia or bacteremia, as these conditions can rapidly progress and become life-threatening.
• Kidney Stones with Infection: The patient's dysuria and urinalysis results could also be consistent with a kidney stone causing a urinary tract obstruction and subsequent infection.

Rare Diagnoses

• Brucellosis: This zoonotic infection can cause a wide range of symptoms, including fever, diarrhea, and urinary symptoms. Although rare, it should be considered in patients with exposure to animals or contaminated animal products.
• Leptospirosis: This bacterial infection can cause a variety of symptoms, including fever, diarrhea, and rash. It is typically associated with exposure to contaminated water or soil.
• Hemolytic Uremic Syndrome (HUS): Although rare, HUS is a potentially life-threatening condition that can cause renal failure, hemolytic anemia, and thrombocytopenia. It is often associated with certain bacterial infections, such as E. coli O157:H7.