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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 2-month-old boy is brought to the emergency department due to irritability and vomiting. The patient is exclusively breastfed but has not been tolerating feeds since yesterday. Urine output has decreased. The mother has a history of obesity and had gastric bypass surgery several years prior to pregnancy. Temperature is 37 C (98.6 F), pulse is 190/min, and respirations are 60/min. The patient is in moderate respiratory distress and has an enlarged liver. Physical examination is otherwise normal. Chest radiography reveals cardiomegaly. Further workup shows impaired carbohydrate metabolism with increased serum levels of lactate and decreased erythrocyte transketolase activity. Which of the following additional enzymes is most likely to have impaired activity in this
patient?
A. Alpha-1,4-glucosidase
B. Alpha-L-iduronidase
C. Galactose-1-phosphate uridyltransferase
D. Glucose-6-phosphatase
E. Pyruvate dehydrogenase
2. A 5-month-old boy is brought to the office due to poor feeding. His mother says that he has difficulty holding his head up while breastfeeding and his suckling seems weaker than usual. Weight is at the 5th percentile. Length and head circumference are tracking along the 25th percentile. Physical examination shows hepatomegaly and hypotonia in all 4 limbs. Cardiac auscultation shows a gallop rhythm, and chest x-ray reveals severe cardiomegaly. Muscle biopsy shows enlarged lysosomes containing periodic acid-Schiff (PAS)-positive material. Which of the following enzymes is most likely deficient in this patient?
A. Acid alpha-glucosidase
B. Galactokinase
C. Glucose-6-phosphatase
D. Glycogen debrancher enzyme
E. Glycogen phosphorylase
F. Pyruvate kinase
3. A 34-year-old previously healthy man comes to the emergency department due to a 3-hour history of chest pain, diaphoresis, and dyspnea. He does not smoke, exercises regularly, and eats a balanced diet. His father died at age 56 from a myocardial infarction. His blood pressure is 110/70 mm Hg and pulse is 110/min and regular. Physical examination is unremarkable. ECG shows ST elevation in the anterolateral leads. Coronary angie>gram reveals proximal left anterior descending artery stenosis and thrombosis, which is treated with angioplasty and stent placement. Laboratory results are as follows:
Further testing reveals a homozygous mutation in the methylene tetrahydrofolate reductase gene that leads to decreased enzymatic activity. Due to this defect, the patient most likely has impairment converting homocysteine to which of the following?
A. Cystathjonine
B. Cysteine
C. Methionine
D. Methylmalonyl-CoA
E. Succinyl-CoA
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