Free Online USMLE Step 1 Questions and Answers 7

4. A 43-year-old man is evaluated for progressive neuropsychiatric symptoms. A year ago, he began feeling depressed and having hallucinations. Five months later, he developed intermittent paresthesias and progressively worsening choreiform movements, myoclonus, and ataxia. These symptoms have not improved despite multiple hospitalizations; an extensive workup has been unrevealing. The patient is a slaughterhouse worker with extensive exposure to bovine offal. As part of the evaluation for prion disease, a tissue sample lysate is processed via gel electrophoresis and transferred to filter paper. Antibodies to a specific prion protein are added to the filter. Next, a marked protein that combines with the antibody-protein complex is used to determine whether the test is positive. Which of the following best describes this test?

A. Microarray
B. Northern blot
C. Southern blot
D. Southwestern blot
E. Western blot

Correct Answer: E

Answer Explanation:

(Choice B) Northern blots analyze mRNA. A sample containing a large number of mRNA molecules is separated via gel electrophoresis. Separated bands are then transferred to a membrane and hybridized with a probe containing a nucleotide sequence complementary to the mRNA of interest.
(Choice C) Southern blotting is used to analyze DNA sequences. DNA that is fragmented using restriction endonucleases is separated by gel electrophoresis and transferred to a nitrocellulose membrane. A radiolabeled DNA probe containing a sequence complementary to an area of interest is then used for hybridization. Restriction site mutations can be detected by Southern blotting because they alter DNA fragment lengths, thereby altering electrophoresis migration patterns. Microarray analysis is similar to Southern and Northern blotting but involves hybridization of a large number of probes at once (Choice
A). The genomic DNA or cDNA being analyzed is labeled with a fiuorescent tag and placed on a gene chip containing complementary sequences for a large number of genes. The degree of fluorescence corresponds to the mRNA expressed in the particular sample.
(Choice D) Southwestern blotting is a technique that analyzes DNA-binding proteins using principles of the Southern and Western blot techniques. DNA-binding proteins are recognized by their ability to bind specific oligonucleotide probes.

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5. A healthy 34-year-old coal mine worker is trapped underground following partial collapse of an access shaft. Rescue efforts are directed toward clearing the obstructed tunnel, but it takes 2 days to reach him. While being taken to the surface, the miner tells rescuers that he feels dizzy and weak. He had an emergency supply of water but has not eaten anything for over 30 hours. Fingerstick blood glucose concentration is 78 mg/dl. Which of the following biochemical reactions is most likely responsible for maintaining this patient’s current blood glucose levels?

A. Acetoacetyl CoA → 3-hydroxy-3-methylglutaryl-CoA
B. Acetyl CoA → palmitic acid
C. Fructose 6-phosphate → fructose 1,6-bisphosphate
D. Glycogen → glucose-1-phosphate
E. Oxaloacetate → phosphoenolpyruvate

Correct Answer: E

Answer Explanation:

(Choice A) Conversion of acetoacetyl-CoA to 3-hydroxy-3-methylglutaryl-CoA occurs during the synthesis of cholesterol and ketone bodies. Ketone body synthesis is increased in starvation situations, however, ketone bodies cannot be used to synthesize glucose.
(Choice B) Palmitic acid is the first fatty acid produced from acetyl CoA during lipogenesis in the fed state. However, during prolonged fasting, lipolysis predominates and leads to generation of glycerol and fatty acids.
(Choice C) Conversion of fructose 6-phosphate to fructose 1,6-bisphosphate occurs during glycolysis and is catalyzed by phosphofructokinase. During starvation, glycolysis is minimized and gluconeogenesis predominates.
(Choice D) The first step of glycogenolysis is breakage of 1-4 glycosidic linkage to form glucose-1-phosphate. After 24 hours of fasting, maintenance of blood glucose levels is achieved mostly through gluconeogenesis, not by glycogenolysis.

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6. A man and woman come to the clinic for preconception counseling. The woman and her parents are healthy, but she has 2 siblings with a rare autosomal recessive disease. The woman’s genotype is unknown. The man is also healthy and has no family history of the disease. It is known that, among healthy individuals in the general population, the carrier frequency is 1 in 200. What is the probability of this couple conceiving a child with the disease?

A. 1/200
B. 1/400
C. 1/800
D. 1/1,200
E. 1/1,600

Correct Answer: D

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