**Question:** In Addison's disease, most diagnostic test is -

A. ACTH stimulation test
B. Plasma cortisol assay
C. Serum electrolyte profile
D. Urine free cortisol assay

**Correct Answer:** D. Urine free cortisol assay

**Core Concept:**
Addison's disease, also known as primary adrenal insufficiency, is a condition where the adrenal glands do not produce sufficient amounts of cortisol and aldosterone due to damage to the adrenal cortex. Cortisol is a vital hormone produced by the adrenal cortex that helps regulate the body's response to stress, blood glucose levels, and immune system function. Aldosterone regulates electrolyte balance, specifically maintaining sodium and potassium levels.

**Why the Correct Answer is Right:**
Urine free cortisol assay is the most suitable diagnostic test for Addison's disease because it directly measures cortisol levels in the urine, providing a more accurate representation of cortisol production than plasma cortisol assay. The plasma cortisol assay only measures cortisol levels in the blood, which may not reflect the adrenal function accurately.

**Why Each Wrong Option is Incorrect:**

A. ACTH stimulation test (Adrenocorticotropic hormone stimulation test): This test measures adrenal response to ACTH stimulation. While useful in evaluating adrenal function, it is not directly related to cortisol production, which is the primary issue in Addison's disease.

B. Plasma cortisol assay: As mentioned before, this test measures cortisol levels in the blood but not in urine, making it less reliable for assessing adrenal function in Addison's disease.

C. Serum electrolyte profile: This test checks electrolyte levels in the blood, which may be abnormal in Addison's disease but does not directly measure cortisol production.

**Clinical Pearl:**

Diagnosing Addison's disease often involves a combination of clinical examination, laboratory tests, and imaging studies (e.g., CT scan or MRI) to identify adrenal gland abnormalities. Urine free cortisol assay is crucial for assessing adrenal cortisol production and diagnosing this condition. It allows for a more accurate assessment of cortisol levels, which are significantly reduced in Addison's disease.