If you’re preparing for the United States Medical Licensing Examination® (USMLE®) Step 1 exam, you might want to know which questions are most often missed by test-prep takers. Check out this example from Kaplan Medical, and read an expert explanation of the answer. Also check out all posts in this series.
A 2-year-old boy is brought in by his mother to the physician because she is concerned by how easily he gets sunburned. She states that he cannot be outside for barely five minutes without getting a painful, blistering sunburn. Physical examination shows a well-developed child with dry skin, many freckles, multiple actinic keratoses on the dorsum of his hands, and bloodshot eyes. Neither of the child's parents has ever had similar symptoms.
This child’s condition is most likely caused by a defect in which of the following cellular processes?
A. Base excision repair.
B. Homologous recombination.
C. Mismatch repair.
D. Non-homologous end joining.
E. Nucleotide excision repair.
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The correct answer is E.
Kaplan Medical explains why
Kaplan Medical explains why
This child’s history of severe sunburn with only minimal UV exposure combined with physical exam findings of freckling, xeroderma, actinic keratoses, and bloodshot eyes are all indications that the boy has xeroderma pigmentosum (XP). Patients with XP have an extremely high risk of developing skin cancer (as many as 50% will develop skin cancer by age 10) and blindness (often due to cataracts or corneal ulcerations).
The condition is caused by a defect in one of several enzymes involved in nucleotide excision repair. When DNA is exposed to UV radiation, thymine dimers form between adjacent thymine bases in a DNA strand. Normally, these thymine dimers are recognized, removed, and replaced. However, in the absence of normal XP repair proteins, the dimers remain, increasing the risk of causing mutations during DNA replication and thereby leading to the development of skin cancer. There are multiple genes that can be mutated in XP, but it is not important to learn these groups. Most forms of the disease are autosomal recessive, although some cases do arise de novo.
Treatment involves avoiding further exposure to damaging UV radiation and removing precancerous keratoses to prevent progression. Even with adequate precautions, life expectancy is poor.
Why the other answers are wrong
Why the other answers are wrong
Choice A: Defects in base excision repair are seen in many cancers. In base excision repair, glycosylases (e.g., DNA glycosylase) remove damaged bases, thus creating an apurinic/apyrimidinic (AP) site. An AP endonuclease removes the deoxyribose 5’-phosphate and the resulting gaps in bases are filled by DNA polymerase and re-sealed by ligase. This most commonly occurs during G1.
