Free Online USMLE Step 1 Questions and Answers 8


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Finding free online USMLE Step 1 questions and answers is an essential step for many medical students and professionals preparing for the exam. When preparing for the USMLE Step 1, it is crucial to reinforce knowledge and become familiar with the exam format through extensive practice questions. Fortunately, our websites will offer free USMLE Step 1 questions and answers, aiding candidates in better preparing for this critical medical examination.

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

Correct Answer: C

Answer Explanation:

(Choice A) DNA methylation (only cytosine and adenine) typically inhibits gene transcription. Demethylatio:n or hypomethylation of oncogenes (and hypermethylation of tumor suppressor genes) contributes to the development of some cancers.
(Choice B) DNA cross-linking can be induced by numerous chemical and physical agents, notably alkylating agents used in cancer treatment.
(Choice D) During DNA replication, incorrect base placement can occur, but enzymes scan the newly synthesized DNA strands for mismatched bases, which are then excised and replaced. This process is guided by the presence of adenine methylation (recognized by the enzymes) in the template strand, as the daughter strand remains unmethylated for some time following DNA replication.
(Choice E) DNA damage from exposure to ultraviolet radiation, a non-ionizing radiation, leads to the formation of pyrimidine-pyrimidine dimers (thymine dimers). Ionizing radiation has higher energy (enough to remove an electron), leading to more cell damage.

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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

Correct Answer: C

Answer Explanation:

(Choice A) Ultraviolet radiation causes the formation of pyrimidine-pyrimidine dimers, not purine-purine dimers.
(Choice B) Exposure to ionizing radiation (x-rays and gamma rays) causes double-strand DNA breaks. The fractured ends can be joined by nonhomologous end joining.
(Choice D) Incorrect base substitution occurs during normal DNA replication and can result in mutations if they are not corrected. DNA mismatch repair in certain prokaryotes is guided by hypermethylation of the parent strand, which helps to identify the non-mutated strand for use as a template.
(Choice E) Deamination of DNA bases (eg, cytosine conversion to uracil, adenine to hypoxanthine) can occur spontaneously or secondary to chemical exposure. These errors are corrected by base excision repair. In this process, abnormal bases are recognized and removed by specific glycosylases without disruption of the phosphodiester backbone. The apurinic and apyrimidinic residues are then removed by specific endonucleases and replaced with the correct base by DNA polymerase.

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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

Correct Answer: C

Answer Explanation:

(Choices A, B, D, E, and F) None of the other amino acids are as abundant as glycine in collagen. X often represents praline and Y is often hydroxyproline or hydroxylysine. Proline residues are essential for alpha helix formation because their ring configuration introduces a kink in the polypeptide chain, enhancing the rigidity of the helical structure. Hydroxylysine is necessary for cross-linking, which greatly increases the tensile strength of assembled collagen fibers.

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