A 40-year-old woman presents with sudden breathlessness. Ventilation-perfusion scintigraphy is planned. The radiopharmaceutical injected intravenously for the perfusion component localizes in the lungs by which mechanism?

• A Active transport into alveolar epithelial cells
• B Phagocytosis by alveolar macrophages
• C Capillary blockade by particles larger than pulmonary capillary diameter ✓
• D Binding to angiotensin-converting enzyme on endothelium

Explanation

Perfusion scanning uses Tc-99m macroaggregated albumin (MAA), particles roughly 10 to 90 micrometers in size. After intravenous injection they lodge mechanically in pulmonary capillaries and arterioles, whose diameter is about 7 to 10 micrometers, so the distribution of trapped particles mirrors regional blood flow. Fewer than 1 in 1000 capillaries is occluded and particles biodegrade over hours. ACE-binding describes Tc-99m MAG3 renal handling, not MAA.

Reference: Grainger & Allison's Diagnostic Radiology, 6th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.