Chest radiography is the most appropriate next diagnostic step. This middle-aged patient presented with the insidious onset of the nephrotic syndrome with membranous glomerulopathy (MG) demonstrated on kidney biopsy. MG may be primary (idiopathic) or secondary to other causes. Primary MG is associated with the antibody to the phospholipase A₂ receptor (PLA₂R) on the podocyte surface in up to 80% of patients, and the presence of the antibody supports this diagnosis. Testing for anti-PLA₂R antibodies may be performed on serum or by staining of kidney biopsy tissue. Secondary causes of MG include malignancies (solid organ cancers, especially lung, colon, and breast), autoimmune diseases (such as lupus or mixed connective tissue disease), infections (hepatitis B and C), and medications (penicillamine, gold, and NSAIDs). Evaluation for potential secondary causes should always be undertaken in patients with MG, especially those who are negative for PLA₂R antibodies. Serologic tests for lupus and hepatitis were negative in this patient, and he has not been exposed to potential offending medications. Although MG may be associated with malignancy, an extensive evaluation for cancer is not indicated in patients with a diagnosis of MG beyond age-appropriate cancer screening except for those with symptoms suggestive of a cancer diagnosis (such as weight loss or blood in the stool) or significant risk factors for specific malignancies. Because of this patient's significant smoking history and his associated increased risk for lung cancer, chest radiography should be obtained as part of his evaluation for secondary MG.

Both PET and PET coupled with CT are useful studies in evaluating selected patients with known malignancy, often for staging purposes. However, neither is indicated to initially evaluate for lung cancer. Similarly, whole body CT is of low yield in searching for malignancy.

Although an extensive evaluation for malignancy is usually not indicated in patients with MG beyond age-appropriate cancer screening, this patient's increased risk for lung cancer warrants further evaluation.