What is the most important investigation for a pediatric patient presenting with poor feeding, hematuria, a large abdominal mass, and decreased air entry in multiple lung areas?

Most Important Investigation: Abdominal Ultrasound

Abdominal ultrasound is the most important initial investigation for this child presenting with poor feeding, hematuria, a large abdominal mass, and decreased air entry in multiple lung areas. 1, 2, 3

Clinical Reasoning

This presentation strongly suggests Wilms tumor (nephroblastoma) with possible pulmonary metastases, fundamentally changing this from a simple hematuria evaluation to a suspected malignancy requiring urgent staging. 1, 2

Why Abdominal Ultrasound Takes Priority

Ultrasound must be performed first to establish the critical foundational information before any other investigation:

• Confirms the renal origin of the abdominal mass versus other abdominal pathology (hepatoblastoma, neuroblastoma, etc.) 1, 3
• Assesses tumor size and extent, including local invasion 1, 3
• Evaluates for inferior vena cava involvement, which impacts surgical planning 1, 3
• Examines the contralateral kidney for bilateral disease (occurs in 5-10% of Wilms tumor) 1, 3
• Requires no ionizing radiation, anesthesia, or contrast administration 1, 4
• Can be performed immediately regardless of the child's clinical status 4

Sequential Diagnostic Algorithm

The American College of Radiology provides clear guidance on the proper sequence: 1, 3

1. First: Abdominal ultrasound - characterizes the mass and confirms renal origin 1, 3
2. Second: Chest CT - performed urgently after ultrasound confirms renal mass for staging of pulmonary metastases 1, 3
3. Third: Consider contrast-enhanced abdominal CT or MRI - defines local extent and vascular invasion after initial characterization 1

Why NOT the Other Options

Lung CT (Option A) - While the decreased air entry suggests pulmonary metastases requiring chest CT for staging, this investigation must follow ultrasound confirmation of the renal mass, not precede it. 1, 3 Proceeding directly to CT without ultrasound is inappropriate because ultrasound may reveal the mass is not renal, completely changing the diagnostic approach. 3

Bone marrow aspiration (Option B) - This is not part of the standard Wilms tumor workup and would only be considered for neuroblastoma or leukemia, which are less likely given the presentation. 1, 3 The abdominal mass must be characterized first before pursuing invasive procedures.

Critical Pitfalls to Avoid

• Never delay ultrasound by assuming this is isolated hematuria requiring no imaging - the presence of an abdominal mass fundamentally changes the clinical scenario from benign hematuria to suspected malignancy. 2
• Never proceed directly to CT without ultrasound confirmation of renal origin, as this exposes the child to unnecessary radiation if the mass proves non-renal. 2, 3
• Never assume isolated hematuria when an abdominal mass is palpable - this is a critical red flag requiring urgent evaluation. 2

Supporting Evidence

The ACR Appropriateness Criteria explicitly state that ultrasound is the initial imaging modality for evaluating abdominal masses in children with hematuria, as it effectively displays kidney anatomy and screens for structural lesions. 5 A renal or bladder mass detected by ultrasound requires further imaging with CT or MRI to define local extent and detect metastases. 5

In children with abdominal masses, ultrasound is usually able to provide accurate localization to a specific organ and differentiate solid from cystic masses. 4 The most common causes of renal masses in children arise from the kidney and urinary tract, making ultrasound the logical first step. 4