Appropriate Investigation for Cushing Syndrome
The appropriate investigation is C: Low-dose dexamethasone suppression test, as this patient presents with the classic triad of Cushing syndrome—proximal muscle weakness, hypertension, and pathognomonic wide purplish (violaceous) striae on the abdomen.
Clinical Presentation Analysis
This 38-year-old man demonstrates highly specific features of hypercortisolism:
- Wide violaceous striae (>1 cm) on the abdomen are pathognomonic for Cushing syndrome and represent a highly specific clinical feature that should immediately prompt evaluation for hypercortisolism 1, 2
- Proximal muscle weakness for 3 months is a key distinguishing feature of Cushing syndrome, manifesting as difficulty with activities requiring proximal muscle strength 1, 2
- Hypertension (140/90 mmHg) is a common comorbidity present in Cushing syndrome 2
Recommended Screening Test
The overnight 1-mg dexamethasone suppression test is the recommended initial screening test for suspected Cushing syndrome based on this clinical presentation 1, 2. The ACC/AHA guidelines specifically list this as the screening test for Cushing syndrome in patients presenting with rapid weight gain, proximal muscle weakness, and wide violaceous striae 1.
Alternative Screening Options
While the dexamethasone suppression test is most appropriate given the answer choices, other acceptable initial screening tests include 2:
- 24-hour urinary free cortisol excretion (preferably multiple collections)
- Midnight salivary cortisol measurement
- Late-night salivary cortisol
Why Other Options Are Less Appropriate
- Fasting blood glucose (Option A): While hyperglycemia occurs in Cushing syndrome, it is neither specific nor diagnostic for the condition 2
- Serum electrolyte (Option B): Hypokalemia with metabolic alkalosis is more characteristic of ectopic ACTH production or primary aldosteronism, not typical Cushing syndrome 2
- MRI brain (Option D): This is a confirmatory test performed after biochemical diagnosis of Cushing syndrome to identify pituitary adenomas, not an initial screening investigation 1
- CT abdomen (Option E): This is used for confirmatory imaging after biochemical diagnosis or for evaluating adrenal masses, not as the initial screening test 1
Clinical Pitfalls to Avoid
Do not order imaging studies before biochemical confirmation of hypercortisolism, as this violates the diagnostic algorithm and may lead to unnecessary procedures 1, 2.
Exclude exogenous glucocorticoid use first before proceeding with biochemical testing, as iatrogenic Cushing syndrome is the most common cause overall 2. This includes high-dose inhaled or intranasal corticosteroids 2.
Purple striae are highly specific for Cushing syndrome and should not be confused with the pink/white striae of rapid weight gain, pregnancy, or normal growth 2.