Free Online USMLE Step 1 Questions and Answers 6

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. Molecular biologists studying signal transduction apply an agent to human cells that activates G-proteindependent phospholipase C. Which of the following intracellular substances is most likely to increase immediately after exposure to this agent?

A. Ca2+
B. cAMP
C. cGMP
D. Cl·
E. mRNA
F. NO

Correct Answer: A

Answer Explanation:

(Choices B and C) Intracellular cAMP and cGMP concentrations increase during activation of adenylate or guanylate cyclase second messenger systems, respectively. Levels can also increase following cyclic nucleotide
phosphodiesterase inhibition, as seen on exposure to sildenafil, which selectively inhibits cGMP phosphodiesterease and results in smooth muscle relaxation in blood vessels.
(Choice D) Intracellular Cl· concentration increases slightly after inhibitory neurotransmitters (eg, GABA, glycine) act on the neuron to increase Cl· membrane conductance (hyperpolarization).
(Choice E) The intracellular concentration of mRNA increases during cellular states of elevated protein synthesis (eg, during cell division).
(Choice F) Nitric oxide (NO) is a paracrine signaling molecule with a lifetime of a few seconds. It can freely cross cell membranes and functions as a critical component of endothelium-mediated vasodilation. NO is synthesized from arginine and 02 by the enzyme NO-synthase.

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2. Researchers studying molecular biology observe a eukaryotic cell via electron microscopy. During interphase of the cell cycle, they notice 10-nm thick chromatin fibers with a “beads on a string” appearance. These chromatin fibers are extracted and treated with an endonuclease, which preferentially cleaves the “string” portions of the chromatin. Further evaluation reveals that the “beads” are composed of DNA wrapped around a core of proteins. Which of the following proteins is most likely found outside this core and promotes chromatin compaction?

A. Histone H1
B. Histone H3
C. Histone H4
D. Small nuclear ribonucleoprotein
E. Type II topoisomerase
F. Ubiquitin

Correct Answer: A

Answer Explanation:

(Choices Band C) Histones H3 and H4 are components of the nucleosome core.
(Choice D) Small nuclear ribonucleoproteins help splice out intrans from pre-mRNA, forming mature mRNA.
(Choice E) During DNA replication, topoisomerase relieves the tension created during DNA strand unwinding by introducing negative supercoils into the DNA. It does not promote chromatin compaction.
(Choice F) Ubiquitin is a small protein present in the cytoplasm and nucleus of all eukaryotes. It is typically covalently attached to various intracellular proteins to signal for their degradation by the proteasome (ubiquitinproteasome pathway).

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3. A 47-year-old homeless man comes to the emergency department due to a “pins-and-needles” sensation in his legs. He also has painful lesions on his lips and comers of his mouth. He has had no loss of consciousness, nausea, vomiting, or diplopia. The patient drinks alcohol heavily on a daily basis and has a history of intravenous heroin use. On physical examination, he appears unkempt and ill appearing. Temperature is 36.8 C (98.2 F), blood pressure is 146/90 mm Hg, and pulse is 106/min. He has glossitis and angular stomatitis. Abdominal examination reveals hepatomegaly. Laboratory evaluation shows very low urinary riboflavin excretion. Activity of which of the following enzymes is most likely directly impaired in this patient?

A. Fumarase
B. Glucose6-phosphate dehydrogenase
C. HMG-CoA reductase
D. lsocitrate dehydrogenase
E. Malate dehydrogenase
F. Succinate dehydrogenase
G. Succinate thiokinase

Correct Answer: F

Answer Explanation:

(Choices A, D, E, and G) lsocitrate dehydrogenase, succinate thiokinase, malate dehydrogenase, and fumarase are enzymes that participate in the TCA cycle but do not use FAD or FMN as cofactors.
(Choice B) Reduced glutathione is an antioxidant that minimizes oxidative damage in many cells. Glutathione reductase regenerates reduced glutathione using nicotinamide adenine
dinucleotide phosphate (NAD PH) as an electron donor and FAD as a cofactor. Although glucose-6-p hosphate dehydrogenase (G6PD) is the rate-limiting enzyme in the pentose phosphate pathway and supplies the necessary NADP H, it does not use FAD as a cofactor.
(Choice C) HMG-CoA reductase is the rate-limiting enzyme in the cholesterol synthesis pathway. FMN and FAD are not used as cofactors.

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