## **Core Concept**
Cushing's disease is a form of Cushing's syndrome caused by excess levels of cortisol due to an adrenocorticotropic hormone (ACTH)-secreting pituitary tumor. The confirmatory test for Cushing's disease aims to distinguish it from other causes of Cushing's syndrome and to localize the source of ACTH overproduction.

## **Why the Correct Answer is Right**
The correct answer, **D. Inferior petrosal sinus sampling (IPSS)**, is considered the gold standard for confirming the diagnosis of Cushing's disease by directly measuring ACTH levels in the inferior petrosal sinuses, which drain blood from the pituitary gland. This test can help lateralize the source of ACTH production within the pituitary gland. It involves measuring ACTH levels in blood samples taken from the inferior petrosal sinuses and comparing them to peripheral ACTH levels, with or without corticotropin-releasing hormone (CRH) stimulation.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not specify a known diagnostic test for Cushing's disease.
- **Option B:** This option is incorrect as it does not correspond to a recognized confirmatory test for Cushing's disease.
- **Option C:** This option is incorrect because, although imaging studies like MRI may help identify a pituitary tumor suggestive of Cushing's disease, they are not considered confirmatory tests.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that **midnight salivary cortisol** and **dexamethasone suppression tests** are used as screening tests for Cushing's syndrome, but **IPSS** is specifically used to confirm Cushing's disease by demonstrating a gradient of ACTH concentration from the petrosal sinuses to the periphery, indicative of pituitary ACTH overproduction.

## **Correct Answer:** D. Inferior petrosal sinus sampling (IPSS).