A 45-year-old woman presents with the clinical features marked **A** in the diagram—moon face with central obesity and purple striae. She has been on oral corticosteroids for 8 years for management of systemic lupus erythematosus. Her 24-hour urinary free cortisol is elevated, and overnight 1-mg dexamethasone suppression test shows a morning cortisol of 2.5 μg/dL. Which of the following is the most likely etiology of her Cushing syndrome?
A. Adrenal adenoma with autonomous cortisol production
B. Exogenous glucocorticoid administration (iatrogenic)
C. ACTH-secreting pituitary adenoma (Cushing disease)
D. Ectopic ACTH secretion from small cell lung cancer
Explanation
Why Exogenous glucocorticoid administration is right
The clinical presentation marked A (moon face, central obesity, purple striae) represents classic Cushing syndrome. The key clinical anchor is that exogenous (iatrogenic) glucocorticoid use is by far the MOST COMMON cause of Cushing syndrome overall, accounting for the majority of cases. This patient has been on chronic oral corticosteroids for 8 years for SLE—a common indication for prolonged steroid therapy in Indian clinical practice. The biochemical confirmation (elevated 24-hour UFC and failure of dexamethasone suppression) confirms hypercortisolism, which in the context of documented chronic steroid use makes exogenous Cushing syndrome the diagnosis. Per the Endocrine Society Cushing Guidelines 2024, exogenous causes far exceed endogenous causes in prevalence.
Why each distractor is wrong
ACTH-secreting pituitary adenoma (Cushing disease): While this is the most common endogenous cause (70% of endogenous Cushing), it accounts for only ~15% of all Cushing syndrome cases overall. The clinical history of chronic steroid use makes exogenous etiology far more likely. Cushing disease would require elevated or normal ACTH levels and would not have an obvious iatrogenic trigger.
Ectopic ACTH secretion from small cell lung cancer: Ectopic ACTH represents only ~10% of endogenous Cushing and would be rare without a known malignancy or suggestive imaging findings. The patient's clear history of chronic steroid use makes this diagnosis unlikely and would require additional investigation (chest imaging, elevated ACTH) to support it.
Adrenal adenoma with autonomous cortisol production: Adrenal adenoma accounts for ~20% of endogenous Cushing (ACTH-independent). However, this patient has an obvious iatrogenic cause (chronic corticosteroid therapy), making an autonomous adrenal lesion an unnecessary and less likely diagnosis. ACTH-independent disease would show suppressed ACTH levels, which would need to be documented.
High-YieldNEET PG
Exogenous (iatrogenic) Cushing syndrome from chronic steroid use is the MOST COMMON cause overall; always take the clinical history of steroid exposure seriously before pursuing endogenous workup.
Endocrine Society Cushing Guidelines 2024
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