Amlodipine-induced edema: Amlodipine, a calcium channel blocker, is known to cause peripheral edema in a significant number of patients. The timing of the edema onset, approximately one month after starting amlodipine, supports this diagnosis. The absence of pain, dyspnea, or shortness of breath and unremarkable labs except for an elevated BNP also points towards amlodipine-induced edema rather than a more severe cardiac issue.

Heart Failure (HF): Although the patient does not report dyspnea or shortness of breath, the elevated BNP level (359) suggests some degree of cardiac dysfunction. Heart failure, especially with preserved ejection fraction, could present with peripheral edema as a primary symptom.
Chronic Venous Insufficiency: This condition is common in the elderly and can cause bilateral lower leg edema due to the incompetence of venous valves, leading to fluid accumulation in the legs.
Nephrotic Syndrome: Although the creatinine and BUN are at baseline, nephrotic syndrome can cause significant proteinuria, leading to hypoalbuminemia and subsequent edema. However, the absence of other symptoms like foamy urine makes this less likely.

Deep Vein Thrombosis (DVT): Although the patient reports no pain, DVT can sometimes present with edema alone, especially in the elderly. Missing this diagnosis could be catastrophic due to the risk of pulmonary embolism.
Lymphoma or Other Malignancies: Certain malignancies can cause edema due to obstruction of lymphatic or venous return. While less common, these diagnoses are critical not to miss due to their significant implications for treatment and prognosis.
Thyroid Disease: Both hypothyroidism and hyperthyroidism can cause edema, though it's less common. Given the potential for significant morbidity if left untreated, thyroid function tests might be warranted.

Sarcoidosis: This autoimmune disease can cause edema among a myriad of other symptoms due to granulomatous infiltration of various organs, including the lymphatic system.
Fabry Disease: A rare genetic disorder that can lead to edema due to the accumulation of globotriaosylceramide in the vascular endothelium, leading to vascular dysfunction.
Lipedema: A chronic condition characterized by the abnormal growth of fat cells in the legs, leading to swelling and edema. It's more common in women but can occur in men, especially with hormonal imbalances.

Expected management would involve:

Discontinuation or substitution of amlodipine if suspected as the cause of edema.
Further cardiac evaluation (e.g., echocardiogram) to assess for heart failure, especially given the elevated BNP.
Venous Doppler studies to rule out DVT.
Laboratory tests to evaluate for nephrotic syndrome, thyroid disease, and other potential causes.
Consideration of a lymphatic or vascular specialist for chronic venous insufficiency or other rare causes of edema.