Lymphedema: This condition is characterized by nonpitting edema due to impaired lymphatic drainage. Given the normal BNP, venous duplex, and echocardiogram, lymphedema becomes a more prominent consideration as it directly affects the lymphatic system without impacting cardiac function or venous competency.

Chronic Venous Insufficiency with Lipodermatosclerosis: Although the venous duplex is normal, chronic changes in the skin and subcutaneous tissue can lead to nonpitting edema. This condition might not always be detected by a duplex scan, especially if the issue is more related to chronic skin and tissue changes rather than acute venous obstruction.
Hypothyroidism: This condition can cause nonpitting edema due to the deposition of mucopolysaccharides in the skin. It's a systemic condition that could explain the edema without affecting the heart or venous system directly.
Sarcoidosis: This is a systemic disease that can cause nonpitting edema among other symptoms. It might involve the lymphatic system or cause skin changes leading to edema.

Nephrotic Syndrome: Although less likely given the nonpitting nature of the edema, nephrotic syndrome can cause significant edema due to hypoalbuminemia. Missing this diagnosis could lead to severe consequences, including renal failure.
Superficial Thrombophlebitis or Deep Vein Thrombosis (DVT) with Non-Compressive Characteristics: While the venous duplex is normal, there's a small chance of missing a DVT, especially if it's non-compressive or located in a less common area. This would be catastrophic if missed due to the risk of pulmonary embolism.
Malignancy-Related Lymphedema: Certain cancers can cause lymphedema by obstructing lymphatic vessels. Missing a malignancy could have severe consequences.

Filarial Infection (Lymphatic Filariasis): In endemic areas, this parasitic infection can cause lymphedema. It's rare in many parts of the world but should be considered in travelers or immigrants from endemic areas.
Fabry Disease: A genetic disorder that can lead to lymphedema among other systemic symptoms. It's rare and might require specific testing for diagnosis.
Lipedema: A condition characterized by the abnormal growth of fat cells in the legs, leading to nonpitting edema. It's often mistaken for lymphedema but has distinct characteristics and requires different management.