USMLE® Step 1 Question of the Day: Chronic cough

Today’s USMLE® Step 1 question of the day features a patient with a chronic cough. What’s the most likely cause? Let’s find out!

A patient undergoes a pulmonary lymph node biopsy to evaluate a chronic cough with the histologic finding demonstrated below.

Which of the following pathological processes likely preceded the development of this histologic finding? 

A. Infection with Mycobacterium tuberculosis

B. Beryllium exposure

C. Silica exposure

D. CD4+ helper T-cell response to unknown antigen

E. Acute inflammation surrounding a foreign body

Scroll down for the correct answer!

The correct answer to today’s USMLE® Step 1 Question is…

A. Infection with Mycobacterium tuberculosis

Correct: See Main Explanation.

Incorrect Answer Explanations

B. Beryllium exposure

Incorrect: Beryllium, an alkaline earth metal, can cause chronic restrictive lung disease. Exposure to this element results in non-caseating granulomatous disease.

C. Silica exposure

Incorrect: Chronic silica exposure is implicated in the development of non-caseating granulomatous disease.

D. CD4+ helper T-cell response to unknown antigen

Incorrect: This describes the proposed mechanism of sarcoidosis, a non-caseating granulomatous disease.

E. Acute inflammation surrounding a foreign body

Incorrect: In general, foreign bodies develop non-caseating granulomas; however, the pathological image obtained from the patient demonstrates a caseating granuloma.

Main Explanation

The above histologic image shows a caseating granuloma (note the area of central necrosis, where no nucleoli can be seen). Of the answer choices listed, only prior infection with tuberculosis is known to induce the formation of caseating granulomas.

Caseating Granuloma	Noncaseating Granuloma
Reproduced from: Wikipedia	Reproduced from: Flikr

Granulomatous inflammation, a subtype of chronic inflammation, refers to an immune response where the body attempts to isolate foreign substances that it is otherwise unable to eliminate. Histologically, it is characterized by epithelioid histiocytes (macrophages with abundant pink cytoplasm) surrounded by giant cells and a rim of lymphocytes. Granulomatous inflammation gets classified into caseating and non-caseating forms (where an area of central necrosis histologically characterizes the latter). In general, non-caseating granulomas are more commonly due to autoimmune disease, whereas caseating granulomas arise from underlying infectious etiologies. The table below outlines diseases that encompass both caseating and noncaseating forms.

Major Takeaway 

Caseating granulomas are histologically characterized by epithelioid histiocytes (macrophages with abundant pink cytoplasm) surrounded by giant cells and a rim of lymphocytes and an area of central necrosis.

Want to learn more about this topic?

Watch this Osmosis video: Tuberculosis: Pathology review

References

• Hoda, Syed A, and Esther Cheng. “Robbins Basic Pathology.” American Journal of Clinical Pathology 148.6 (2017): 557–557. Web.

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