Differential Diagnosis for Swelling Below Both Knees in a 17-Year-Old Girl

Single Most Likely Diagnosis

• Lymphedema: This condition is characterized by swelling in one or more extremities, resulting from impaired flow in the lymphatic system. The low zinc levels could be related to impaired lymphatic function, as zinc plays a role in maintaining healthy tissues, including those in the lymphatic system. The absence of trauma or other comorbidities and the presence of low zinc and less iron in the blood make lymphedema a plausible diagnosis, especially if the swelling is chronic and not associated with other systemic symptoms.

Other Likely Diagnoses

• Protein-Energy Malnutrition: Although less common in developed countries, protein-energy malnutrition can lead to edema due to hypoalbuminemia. The low zinc and iron levels could indicate a broader nutritional deficiency, contributing to the swelling.
• Hypothyroidism: This condition can cause non-pitting edema (myxedema) and is associated with various metabolic and nutritional deficiencies, including low zinc levels. However, other symptoms of hypothyroidism would typically be present.
• Sarcoidosis: A systemic granulomatous disease that can cause lymphadenopathy and, less commonly, lymphedema. It might also lead to abnormalities in mineral levels, including zinc.

Do Not Miss Diagnoses

• Deep Vein Thrombosis (DVT): Although less common in young individuals without a history of trauma or comorbidities, DVT can cause swelling in one or both legs and is a medical emergency due to the risk of pulmonary embolism.
• Lymphoma: Certain types of lymphoma can cause lymphadenopathy and subsequent lymphedema. Given the potential severity of lymphoma, it's crucial to consider this diagnosis, even if it seems less likely.
• Infection: Chronic infections, such as cellulitis or lymphangitis, can cause swelling and might be associated with systemic signs of infection, although the question specifies all other parameters are normal.

Rare Diagnoses

• Klippel-Trenaunay Syndrome: A rare congenital malformation that involves blood and lymph vessels, leading to edema, varicose veins, and other vascular anomalies.
• Milroy Disease: A rare, inherited condition characterized by chronic lymphedema, typically presenting at birth or during infancy, but could potentially be diagnosed later in life if mild.
• Fabry Disease: An X-linked lysosomal storage disorder that can cause edema among a myriad of other systemic symptoms due to the accumulation of globotriaosylceramide in various cells.

Each of these diagnoses should be considered in the context of the patient's full clinical presentation, and further diagnostic testing would be necessary to determine the underlying cause of the swelling.