Endocrine Pharmacology USMLE Step 1 Practice Question

A 24-year-old woman with autoimmune primary adrenal insufficiency on hydrocortisone 20 mg daily presents with orthostatic dizziness and intense salt craving for two weeks. Vital signs show BP 92/58 mmHg supine with a further drop to 78/50 mmHg standing, HR 108/min, RR 16/min, temperature 37°C, and SpO2 98%. Physical examination reveals dry mucous membranes. Laboratory studies show serum sodium 128 mEq/L, potassium 6.2 mEq/L, and elevated plasma renin activity. Thyroid function is normal. Which drug should be added to address her mineralocorticoid deficiency?