For a patient with rheumatoid arthritis presenting with lower leg edema and proteinuria, what further investigation is most likely to provide diagnostic value?

In the context of a patient with rheumatoid arthritis who is experiencing lower leg edema and proteinuria, the most relevant investigation to consider is serum protein electrophoresis. This test allows for the evaluation of different types of proteins in the blood, including their concentrations and patterns, which can help identify conditions such as nephrotic syndrome or multiple myeloma, both of which may present with the symptoms described.

In rheumatoid arthritis, the development of renal involvement can occur due to various reasons, including secondary complications like amyloidosis or even medication side effects. Serum protein electrophoresis can assist in identifying abnormal protein levels, such as monoclonal proteins that may point towards a hematological malignancy or inflammation-related conditions.

Lower leg venography is typically utilized to assess for deep venous thrombosis, which does not specifically correlate with the symptoms of proteinuria seen in this patient. A rectal biopsy is not relevant in this scenario as it is used primarily for gastrointestinal conditions. Similarly, renal angiography does not provide direct insight into the protein composition in the urine and is less commonly employed in the evaluation of proteinuria associated with rheumatoid arthritis. Thus, serum protein electrophoresis stands out as the most diagnostically valuable step in this clinical situation.