Differential Diagnosis for a 6-week-old Male with Fever, Vomiting, Diarrhea, and Fussiness

Single Most Likely Diagnosis

• Urinary Tract Infection (UTI): The presence of three organisms in the urine culture, including Escherichia coli, Klebsiella pneumoniae, and Enterococcus faecium, with significant colony counts, especially in a straight catheter specimen, strongly suggests a UTI. The clinical presentation of fever, vomiting, and fussiness in a 6-week-old male also supports this diagnosis.

Other Likely Diagnoses

• Gastroenteritis: The symptoms of vomiting and diarrhea could indicate a gastrointestinal infection, which is common in infants. However, the presence of a significant UTI might be the primary cause of these symptoms.
• Sepsis: Although the blood work does not strongly indicate sepsis at this point (e.g., the WBC count is slightly low), the clinical presentation and the presence of multiple organisms in the urine culture could suggest a systemic infection. Sepsis is a concern in infants with UTIs, especially if not promptly treated.
• Dehydration: Given the symptoms of vomiting and diarrhea, dehydration is a likely complication that needs to be addressed.

Do Not Miss Diagnoses

• Meningitis: In infants, especially those under 3 months, any sign of infection (like fever) warrants consideration of meningitis due to the risk of severe consequences if missed. Although the current symptoms and lab results do not directly point to meningitis, it's crucial to consider and potentially rule out with further evaluation if necessary.
• Bacteremia: The presence of multiple organisms in the urine could potentially indicate bacteremia, especially if the patient shows signs of systemic infection. Prompt antibiotic treatment and monitoring are essential.

Rare Diagnoses

• Congenital Anomalies of the Urinary Tract: While not directly indicated by the current information, recurrent or severe UTIs in infancy can sometimes be the first sign of underlying congenital anomalies of the urinary tract. Further investigation might be warranted if UTIs become recurrent.
• Immune Deficiency: Although rare, recurrent severe infections in infancy could suggest an underlying immune deficiency. This would be considered if the infant has recurrent infections that are severe or difficult to treat.