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USMLE® Step 1 Question of the Day: Suboccipital Fat Pad

Osmosis Team
Published on Jun 16, 2021. Updated on Jun 15, 2021.

Each week, Osmosis shares a USMLE® Step 1-style practice question to test your knowledge of medical topics. Today's case involves a 40-year-old man with hyperpigmentation of the skin. Can you figure it out?

A 40-year-old man comes to his outpatient provider with a concern of skin changes that began 4 months ago. The patient has no past medical history. The patient works as an attorney and spends most of his time indoors. During the visit, his temperature is 37.4°C (99.3°F) and blood pressure is 141/84 mmHg. His BMI is 33 kg/m2. Physical examination is notable for a bulge beneath his occiput and purple striae located predominantly in the lower abdomen. The patient is also observed to have hyperpigmentation of the skin. Which of the following is the most likely cause of the patient’s presentation?

A. Overactive posterior pituitary adenoma
B. Overactive anterior pituitary adenoma
C. Hyperfunctioning renin-secreting tumor
D. Hyperfunctioning cortisol-secreting tumor
E. Hyperfunctioning aldosterone-secreting tumor

Scroll down to find the answer!


The correct answer to today's USMLE® Step 1 Question is...

B. Overactive anterior pituitary adenoma

Before we get to the Main Explanation, let's look at the incorrect answer explanations. Skip to the bottom if you want to see the correct answer right away!

Incorrect answer explanations

The incorrect answers to today's USMLE® Step 1 Question are...

A. Overactive posterior pituitary adenoma

Incorrect: The posterior pituitary secretes vasopressin and oxytocin. An overactive posterior pituitary adenoma can result in the excess production of either of these hormones. However, neither excess oxytocin or vasopressin production would account for the striae and sub-occipital bulge (termed a “buffalo’s hump”) noted in this patient.

C. Hyperfunctioning renin-secreting tumor

Incorrect: Overproduction of renin can lead to increased downstream aldosterone production and cause hypertension. However, a renin-secreting tumor would not account for the skin findings observed in this patient. Instead, the abdominal striae and suboccipital hump are most consistent with the presentation of hypercortisolism.

D. Hyperfunctioning cortisol-secreting tumor

Incorrect: A cortisol-secreting tumor can account for many of the patient’s findings. However, the patient’s skin hyperpigmentation is most likely due to ACTH overproduction. This is because melanocyte stimulating hormone (MSH) is cleaved from the same precursor as ACTH. So increased ACTH production would also lead to increased MSH production. In contrast, patients with hypercortisolism due to a cortisol-secreting tumor would have low ACTH levels and no skin pigmentation.

E. Hyperfunctioning aldosterone-secreting tumor

Incorrect: An aldosterone-secreting tumor can cause hypertension. However, an aldosterone-secreting tumor would not account for the patient’s weight gain, skin findings, and suboccipital hump.

Main Explanation

Cushing syndrome is the constellation of findings seen in patients with hypercortisolism. Excess cortisol leads to muscle and skin breakdown. This leads to muscle wasting and thin extremities, easy bruising and abdominal striae (reddish-purple streaks). The bones are also broken down for calcium resorption, so fractures can occur due to osteoporosis. Cortisol also stimulates gluconeogenesis, which results in hyperglycemia, and increased appetite, which results in weight gain. This can manifest as truncal obesity (accumulation of adipose at the abdomen). However, there is also fat redistribution to the cheeks, resulting in a round, moon-shaped face, and to the back of the neck leading to the development of a suboccipital fat pad, or “buffalo hump.” Cushing syndrome can also lead to hypertension since cortisol amplifies the effect of catecholamines and acts as a mineralocorticoid agonist.

Cushing syndrome can be due to high levels of exogenous or endogenous cortisol. The most common cause of Cushing syndrome is chronic corticosteroid use. Endogenous causes include adrenal tumors, malignancies that secrete ectopic ACTH, and overproduction of ACTH by a pituitary adenoma. The latter is termed Cushing disease.

Patients with Cushing disease will have elevated ACTH levels and skin hyperpigmentation. This is because melanocyte stimulating hormone (MSH) is cleaved from the same precursor as ACTH; therefore, increased production of ACTH would also lead to increased production of MSH. In contrast, patients with hypercortisolism due to exogenous corticosteroid use or an adrenal tumor would have low ACTH levels and no skin pigmentation.


Major Takeaway

The patient in this vignette has both Cushing syndrome (hypercortisolism) and Cushing disease (hypercortisolism due to a pituitary adenoma). Symptoms of hypercortisolism include muscle breakdown, striae, hyperglycemia, hypertension, moon-facies, and a buffalo hump.

References

Barbot, M., Zilio, M., Scaroni, C. (2020) Cushing’s syndrome: Overview of clinical presentation, diagnostic tools and complications. Best Practices & Research Clinical Endocrinology & Metabolism. 101380. Doi: 10.1016/j.beem.2020.101380.

Buliman, A., Tataranu, L.G., Paun, D.L., Mirica, A., Dumitrache, C. (2016) Cushing’s disease: a multidisciplinary overview of the clinical features, diagnosis, and treatment. Journal of Medicine and Life. 9(1), 12-18. PMID: 27974908.

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The United States Medical Licensing Examination (USMLE®) is a joint program of the Federation of State Medical Boards (FSMB®) and National Board of Medical Examiners (NBME®). Osmosis is not affiliated with NBME nor FSMB.