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Biochemistry
1. A 23-year-old previously healthy man comes to the office after noticing a painless, hard mass in the left testis. Scrotal ultrasound shows a solid testicular mass, and CT scan of the abdomen and pelvis shows left paraaortic lymphadenopathy. Left orchidectomy is performed and postoperative histopathology reveals seminoma of the testis. External beam radiotherapy is administered to the paraaortic metastatic area. Several weeks later, the retroperitoneal nodes are observed to have markedly decreased in size. Which of the following is the most likely effect of the therapy used on the metastatic cells in this patient?
A. Demethylation of DNA
B. DNA cross-linking
C. Double-strand DNA breaks
D. Nucleotide mismatches
E. Pyrimidine dimers
2. An 18-year-old man comes to the urgent care cl inic due to painful erythema affecting his extremities, trunk, and face. He is vacationing in Florida and spent 5 hours at a Gulf coast beach earlier in the day. The patient did not apply any sunscreen as it was cloudy. The physician explains that cloud cover does not afford a high degree of protection against the sun, especially with prolonged exposure in highly reflective environments like the beach. He is advised to wear protective clothing and apply sunscreen to prevent recurrence. Which of the following is most likely to happen within the patient’s skin cells as a result of his exposure?
A. Covalent bond formation between adjacent purine bases
B. End-joining repair of double-stranded DNA breaks
C. Endonuclease nicking of the damaged DNA strand
D. Hypermethylation of residues in the undamaged DNA strand
E. Removal of deaminated bases by glycosylase
3. A dermatology researcher is studying the role of different amino acids in wound healing. She cultures mature dermal fibroblasts in growth media. After several days, the fibroblasts begin synthesizing polypeptide chains that assemble into triple helical structures, followed by fibrils. The fibrillar proteins are hydrolyzed and separated into their constituent amino acids via paper chromatography. Which of the fol lowing amino acids is most likely to be found in highest quantity in these proteins?
A. Alanine
B. Cysteine
C. Glycine
D. Leucine
E. Lysine
F. Proline