USMLE Step 1 Question 304

Question:

A 60-year-old male surfer presents with a 1-inch mass on the right side of his forehead. A biopsy reveals squamous cell carcinoma. Which of the following is responsible for the formation of pyrimidine dimers in DNA, thus leading to squamous cell carcinoma?

Correct Answer: C.  UV-B

Ultraviolet light, particularly UV-B (Answer C), is known to mutate DNA by creating pyrimidine dimers in DNA. It is associated with the formation of skin cancers, including squamous cell carcinoma, basal cell carcinoma, and malignant melanoma. Ultraviolet light is divided into UV-A (320 to 400 nm), UV-B (280 to 320 nm), and UV-C (200 to 280 nm).

UV-C (Answer D), although a potent mutagen, is not significant because it is filtered out by the ozone layer around the earth.

Some DNA viruses and RNA viruses are associated with the development of dysplasia and malignancy. For example, infection with human papillomavirus (HPV), especially types 16 and 18, is associated with cervical dysplasia; Epstein-Barr virus (EBV; Answer B) is associated with Burkitt’s lymphoma and nasopharyngeal carcinoma; hepatitis B virus (HBV) and hepatitis C virus (HCV) are associated with primary hepatocellular carcinoma; and HHV-8 is associated with Kaposi’s sarcoma. HTLV-I is an RNA retrovirus that is associated with the formation of a peculiar type of hematologic malignancy called adult T cell leukemia/lymphoma. These patients have malignant cells in their lymph nodes and blood. This malignancy is endemic in southern Japan and the Caribbean region.

Aflatoxin (choice A) is secreted by Aspergillus flavus and is a suspected liver carcinogen.

Vinyl chloride (choice E) is an industrial chemical used to make polymers. It has been associated with hepatic hemangiosarcomas.