Differential Diagnosis

The patient presents with a normal chest X-ray (CXR), cough, elevated white blood cell count (WBC) of 18, and an ammonia level of 61. Considering these findings, the differential diagnosis can be categorized as follows:

• Single Most Likely Diagnosis
  • Pneumonia not visible on CXR: This could be due to an early infection or a type of pneumonia that does not cause significant consolidation visible on a standard chest X-ray, such as mycoplasma pneumonia or viral pneumonia. The elevated WBC count supports an infectious process.
• Other Likely Diagnoses
  • Urinary Tract Infection (UTI): Although the primary symptoms are respiratory, a UTI could cause an elevated WBC count. The ammonia level could be related to a UTI, especially if the patient has a urinary tract obstruction or neurogenic bladder leading to urinary stasis.
  • Gastroenteritis: This could lead to dehydration and potentially affect ammonia levels indirectly. However, the primary symptom of coughing might not directly correlate unless the gastroenteritis has led to aspiration.
• Do Not Miss Diagnoses
  • Sepsis: Although the CXR is normal, sepsis from any source (including pneumonia, UTI, or another infection) is a life-threatening condition that must be considered, especially with an elevated WBC count.
  • Reye's Syndrome: This rare but serious condition, often associated with viral infections and aspirin use in children, can cause elevated ammonia levels and could present with nonspecific symptoms including cough if there's an associated respiratory infection.
• Rare Diagnoses
  • Hyperammonemia due to Inborn Errors of Metabolism: Conditions like ornithine transcarbamylase deficiency could present with elevated ammonia levels but are rare and typically identified in infancy or early childhood.
  • Cystic Fibrosis: Could present with recurrent respiratory infections and might have an elevated WBC count, but the ammonia level would be unusual unless there was significant liver dysfunction, which is less common in early stages.

Each of these diagnoses requires careful consideration of the patient's full clinical picture, including age, medical history, and additional laboratory or imaging findings.