What is the most important investigation for a child presenting with poor feeding, hematuria, and a large abdominal mass?

Abdominal Ultrasound is the Most Important Initial Investigation

Abdominal ultrasound is the most important investigation for this child presenting with poor feeding, hematuria, and a large abdominal mass, as it serves as the critical first step to confirm the renal origin of the mass, assess bilateral kidney involvement, evaluate for vascular extension, and determine tumor size—all essential for diagnosis and staging of suspected Wilms tumor. 1, 2

Why Abdominal Ultrasound Takes Priority

The American College of Radiology explicitly recommends abdominal ultrasound as the first-line imaging modality for evaluating pediatric patients with a palpable abdominal mass and hematuria. 1, 2 This recommendation is based on several critical advantages:

• Confirms renal origin of the mass, which fundamentally determines the entire diagnostic and treatment pathway 1, 2
• Assesses the contralateral kidney for bilateral involvement, which occurs in 5-10% of Wilms tumor cases and dramatically changes surgical planning 1
• Evaluates for inferior vena cava involvement and tumor thrombus, critical for surgical planning 1
• Requires no ionizing radiation, making it ideal for pediatric patients 3, 1
• Requires no sedation and can be performed rapidly without fasting 3
• Has high sensitivity for detecting renal masses and characterizing their features 3

Clinical Context: This Presentation Suggests Wilms Tumor

The combination of poor feeding, hematuria, and a large abdominal mass in a child raises immediate concern for Wilms tumor (nephroblastoma). 1 The decreased air entry on chest examination suggests possible pulmonary metastases, making rapid diagnosis and staging essential for mortality and morbidity outcomes.

Why the Other Options Are Incorrect

Lung CT (Option A)

While chest CT is absolutely essential for staging after a renal mass is confirmed, proceeding directly to chest CT before confirming the renal origin of the abdominal mass is inappropriate. 1, 2 The ultrasound may reveal the mass is not renal in origin, which would completely change the diagnostic approach. 1, 2 Chest CT should follow urgently after ultrasound confirmation of a renal tumor, as pulmonary metastases significantly impact staging and treatment intensity. 2

Bone Marrow Aspiration (Option B)

Bone marrow aspiration would be more relevant if neuroblastoma were suspected, but this is not the initial investigation even in that scenario. The first step remains imaging to characterize the abdominal mass itself. 3

Sequential Diagnostic Algorithm After Ultrasound

Once ultrasound confirms a renal mass:

1. Proceed urgently to chest CT for staging to evaluate for pulmonary metastases 1, 2
2. Consider contrast-enhanced abdominal CT or MRI to define local extent of disease, vascular invasion, and surgical resectability 1
3. MRI is preferred over CT for further abdominal characterization due to lack of ionizing radiation and superior lesion characterization, particularly for detecting multiple tumors and nephrogenic rests 3

Critical Pitfall to Avoid

Never delay imaging by assuming this represents isolated hematuria requiring no immediate workup. 1, 4 The presence of a palpable abdominal mass fundamentally changes the clinical scenario from isolated hematuria and requires urgent evaluation. 1, 4 Similarly, never proceed directly to CT without ultrasound confirmation of renal origin, as this exposes the child to unnecessary radiation if the mass proves to be non-renal. 1, 2