What is the most important test in disease assessment for a 2-year-old child with hematuria (blood in urine) and an abdominal mass, and reduced air entry on multiple sites?

Abdominal Ultrasound (Option C)

For a 2-year-old child presenting with hematuria, an abdominal mass, and reduced air entry on multiple sites, abdominal ultrasound is the most important initial test for disease assessment, as this clinical triad strongly suggests Wilms tumor with pulmonary metastases, and ultrasound is the recommended first-line imaging to confirm the renal origin of the mass, assess bilateral involvement, evaluate vascular extension, and guide subsequent staging investigations. 1

Clinical Context and Differential Diagnosis

This presentation is highly concerning for Wilms tumor (nephroblastoma), which classically presents with:

• A palpable abdominal mass (most common presenting sign)
• Hematuria (occurs in 15-30% of cases)
• Reduced air entry suggesting pulmonary metastases (lungs are the most common site of metastatic disease) 1

The combination of these three findings fundamentally changes the clinical scenario from isolated hematuria to a suspected renal malignancy requiring urgent evaluation 1, 2.

Why Abdominal Ultrasound is the Priority

Ultrasound should be the first imaging investigation performed for several critical reasons:

• Confirms renal origin of the mass and differentiates it from other abdominal masses (neuroblastoma, hepatoblastoma) 1, 3
• Assesses the contralateral kidney for bilateral involvement, which occurs in 5-10% of Wilms tumor cases 1
• Evaluates inferior vena cava involvement and tumor thrombus extension, which affects surgical planning 1
• Determines tumor size and extent without radiation exposure 1, 3
• Can be performed safely regardless of the patient's clinical status, is noninvasive, painless, requires no contrast media, and is relatively inexpensive 3
• Provides excellent differentiation between solid and cystic masses 3

Sequential Diagnostic Algorithm

1. Immediate abdominal ultrasound of kidneys and bladder to characterize the mass 1, 2

2. Plain abdominal radiography can be performed concurrently to assess for calcifications (more common in neuroblastoma) and provide baseline anatomic information 1, 2

3. After ultrasound confirms a renal mass, proceed urgently to:

   • Chest CT for staging and detection of pulmonary metastases (which explains the reduced air entry) 1, 2
   • Contrast-enhanced abdominal CT or MRI to define local extent, vascular invasion, and lymph node involvement 1, 2

Why Other Options Are Incorrect

Nuclear study (Option A) has no role in the initial assessment of a suspected renal mass in children and would delay critical diagnosis 1.

Lung CT (Option B), while ultimately necessary for staging, should not be performed before confirming the renal origin of the mass with ultrasound. The reduced air entry may be due to metastases, but you must first establish the primary diagnosis 1.

Abdominal X-ray (Option D) provides limited information and cannot adequately characterize the mass or assess for the critical features needed for diagnosis and surgical planning 1.

Critical Pitfalls to Avoid

• Delaying imaging by assuming this represents isolated hematuria requiring no immediate workup is a critical error—the presence of a palpable abdominal mass fundamentally changes management 1, 2

• Proceeding directly to CT without ultrasound confirmation wastes time and radiation if the mass is not renal in origin, which would change the entire diagnostic approach 1

• Ignoring the respiratory findings—reduced air entry in multiple sites with a renal mass strongly suggests pulmonary metastases, but chest imaging should follow, not precede, confirmation of the primary tumor 1

The American College of Radiology explicitly recommends ultrasound as the initial imaging modality for pediatric patients with palpable abdominal mass and hematuria, making this the evidence-based standard of care 1, 2.