Free Online USMLE Step 1 Questions and Answers 4

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 6-month-old boy is brought to the emergency department by his mother because of recent onset of vomiting, irritability, and jaundice. The infant was born at term and had been healthy until the onset of these symptoms. All of his vaccinations are up-to-date. He had been breast-fed exclusively until 1 week ago, when cereals and fruit juices were introduced into his diet. Further evaluation reveals hepatomegaly and abnormal liver function tests. Which of the following enzymes is most likely to be deficient in this patient?

A. Galactose-1-phosphate uridyl transferase
B. Aldolase B
C. Fructokinase
D. Galactokinase
E. Acid α-glucosidase

Correct Answer: B

Answer Explanation:

(Choices A and D) Galactose- 1-phosphate uridyl transferase deficiency (classic galactosemia) is an autosomal recessive disorder characterized by vomiting, feeding intolerance, neonatal jaundice, hepatomegaly, and death if untreated. Symptoms start soon after breastfeeding is initiated. Galactokinase deficiency is a more benign disorder of galactose metabolism that results in the formation
of neonatal cataracts.
(Choice C) Fructokinase deficiency causes essential fructosuria, a benign autosomal recessive disorder. Fructose from the diet is absorbed and secreted freely in the urine due to impairment of the first step in fructose metabolism.
(Choice E) Glycogenolysis is accomplished mainly by glycogen phosphorylase and debranching enzyme, but a small amount is also broken down by the lysosomal enzyme a-1,4-glucosidase. Alpha-glucosidase (or acid maltase) deficiency causes Pompe disease. This disease presents not with hypoglycemia, but with cardiomyopathy and hypotonia.

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2. A 35-year-old man comes to the office with progressively worsening fatigue associated with dark urine and back pain. Two days ago, the patient ate some large, flat beans brought home by his wife after a business trip to Egypt. Physical examination shows jaundice and pallor. Laboratory results reveal a hemoglobin level of 8 g/dl . Further evaluation reveals deficiency of an enzyme involved in the conversion of glucose-6-phosphate to ribulose-5-phosphate. The substance generated during this conversion is necessary for which of the following biochemical processes?

A. ADP phosphorylation
B. Fatty acid synthesis
C. Glycogen storage
D. Ketone body synthesis
E. Protein degradation

Correct Answer: B

Answer Explanation:

(Choice A) NADH can be used as a reducing agent to convert ADP to ATP during oxidative phosphorylation. In contrast to NADH, NADPH cannot be used to convert ADP into ATP.
(Choice C) Glycogenesis is the process by which glucose is stored for later use through the addition of glucose molecules to glycogen chains. It does not require NADPH.
(Choice D) Ketone bodies are formed mainly in the liver during times of fasting when there is increased fat degradation. Cytosolic HMG-CoA synthase is the starting point of cholesterol synthesis whereas the mitochondrial version of the enzyme is the rate-limiting step in ketone body synthesis. Unlike cholesterol synthesis, ketone body production does not require NADPH.
(Choice E) Protein catabolism begins with the hydrolysis of polypeptides into amino acids. These subsequently undergo transamination reactions that funnel the amine nitrogen predominately into glutamate, which is oxidatively deaminated to produce ammonia. The urea cycle then converts ammonia into urea for elimination in the urine.

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3. A 64-year-old man comes to the emergency department due to severe abdominal pain, nausea, and vomiting. Medical history is significant for hypertension, myocardial infarction, and heart failure with reduced ejection fraction. The patient does not use alcohol but smokes a half-pack of cigarettes daily. Temperature is 37.6 C (99.7 F), blood pressure is 100/60 mm Hg, pulse is 116/min and irregular, and respirations are 24/min. The abdomen is soft, mildly distended, and mildly tender without rebound or guarding. Laboratory results are as follows:

Usmle4 1

CT angiography of the abdomen reveals an occlusion within the proximal superior mesenteric artery. Decreased activity of which of the following enzymes best explains this patient’s acid-base disorder?

A. Enolase
B. Lactate dehydrogenase
C. Pyruvate carboxylase
D. Pyruvate dehydrogenase
E. Pyruvate kinase

Correct Answer: D

Answer Explanation:

(Choices A and E) Enolase converts 2-phosphoglycerate to phosphoenolpyruv ate, which is subsequently metabolized by pyruvate kinase into pyruvate. Because these glycolytic reactions can take place under both aerobic and anaerobic conditions, they are not affected by tissue ischemia.
(Choice C) Pyruvate can also be converted to oxaloacetate by pyruvate carboxylase. This enzyme is involved in regeneration of glucose from pyruvate during the process of gluconeogenesis, which typically occurs in a fasted (not ischemic) state.

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