Free Online USMLE Step 1 Questions and Answers 1


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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. Foreign antigen recognition at the cell surface by cytotoxic T-lymphocytes stimulates a response that results in rapid cell death. Which of the following act as effectors of this response?

A. Matrix metalloproteinases
B. Acid hydrolases
C. Caspases
D. Phospholipase A2
E. Protein kinase A

Correct Answer: C

Answer Explanation:

(Choice A) Metalloproteinases are zinc-containing enzymes that degrade the components of the extracellular matrix (eg, collagen, laminin, fibronectin). Metalloproteinases are essential for proper tissue remodeling during wound healing.
(Choice B) Acid hydrolases do not participate in apoptosis.
(Choice D) Phospholipase A2 does not participate in apoptosis.
(Choice E) Protein kinase A is a component of the cAMP-associated signaling system. The binding of a ligand to a G-protein-linked receptor results in adenylyl cyclase activation and the release of cAMP. Elevated levels of cAMP activate protein kinase A.

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2. A 5-day-old boy is diagnosed with hyperphenylalaninemia by newborn screening. He is placed on a special phenylalanine-restricted diet with tyrosine supplementation. The parents are extensively counseled on the patient’s condition and informed of the necessary dietary restrictions. They are also instructed to return to the clinic for regular follow-up visits. Several months later, laboratory results indicate that the infant has a normal serum phenylalanine level. Careful examination, however, shows some neurologic abnormalities, including axial hypotonia. Further work-up is notable for elevated prolactin. Impaired synthesis of a cofactor essential for metabolism of aromatic amino acids is suspected. Which of the following enzymes is most likely deficient in this patient?

A. Dihydropteridine reductase
B. Dopamine beta-hydroxylase
C. PhenylaJanine hydroxylase
D. Phenylethanolamine N-methyltransferase
E. Tyrosinase

Correct Answer: A

Answer Explanation:

(Choices Band D) Dopamine beta-hydroxylase converts dopamine to norepinephrine, and phenylethanolamine N-methyltransferase converts norepinephrine to epinephrine. Deficiency of either enzyme would not result in elevated phenylalanine levels, as seen in this patient’s initial presentation.
(Choice C) Phenylalanine hydroxylase deficiency results in PKU. A normalized phenylalanine level would be expected with a phenylalanine-restricted diet, as seen in this patient, but neurologic manifestations of disease would not progress. Moreover, tyrosine supplementation in PKU allows for dopamine production, and this patient’s hyperprolactinemia suggests persistent dopamine deficiency.
(Choice E) Melanin is synthesized from tyrosine derivatives by tyrosinase in melanocytes. Tyrosinase deficiency (absent melanin production) causes albinism, characterized by a lack of pigment in skin and hair. A history of hyperphenylalaninemia and progressive neurologic findings would not be seen.

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3. A 54-year-old man comes to the emergency department with a 3-month history of fatigue and exertional dyspnea. He has early satiety and frequent upper abdominal discomfort. On physical examination, the patient has palpable splenomegaly but no lymphadenopathy. Laboratory tests are as follows:

Usmle1 1

Reverse transcription polymerase chain reaction is used to diagnose chronic myelogenous leukemia in this patient. Which of the following is most likely to be detected by this test?

A. Chromosomal position of the BCR and ABL genes
B. DNA rearrangement in the BCR promoter region
C. Fusion protein containing BCR and ABL domains
D. Messenger RNA transcript containing BCR and ABL exons
E. Point mutation in the ABL enhancer region

Correct Answer: D

Answer Explanation:

(Choice A) Fluorescence in situ hybridization (FISH) techniques allow direct localization of genes to their respective chromosomes by using a labeled DNA probe complementary to the sequence of interest.
(Choices Band E) RT-PCR amplification uses an mRNA template, so it cannot detect changes in the parts of the gene that are not transcribed (eg, promoter and enhancer regions). Other PCR techniques that use chromosomal DNA can detect changes in these nontranscribed regions.
(Choice C) RT-PCR is used to detect levels of mRNA expression; it does not identify proteins. A Western blot study can detect the BCR-ABL protein by using monoclonal antibodies directed against BCR or ABL.

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