In suspected pulmonary embolism, a V/Q scan is preferred when which scenario is present?

• A. When there is a contrast allergy but no kidney disease.
• B. To diagnose pneumonia.
• C. When PE is suspected but the patient has severe kidney disease.
• D. When PE is suspected after a normal D-dimer.

Answer: (C)

In suspected pulmonary embolism, the imaging choice is guided by safety and diagnostic capability. When a patient has severe kidney disease, iodinated contrast used for CT pulmonary angiography poses a real risk of contrast-induced nephropathy. A ventilation–perfusion scan uses radiotracers without iodinated contrast, so it provides essential diagnostic information for PE while avoiding further kidney injury. That safety advantage makes a V/Q scan the preferred option in this scenario. The other contexts—like a contrast allergy (which would also push away from CT), pneumonia diagnosis, or a normal D-dimer—don’t fit as well for choosing V/Q here; the key point is avoiding nephrotoxic contrast in renal impairment.