Practice Test 10 - USMLE Step 1

20 questions. Answer them in any order, then submit to see your score and every explanation.

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1. What does the plateau phase of this dose-response curve represent?
2. The mutation most likely disrupts which of the following regulatory sequences?
3. The patient's worsening respiratory depression is primarily due to the suppression of which of the following mechanisms?
4. A 40-year-old man is treated with penicillin G for early syphilis. Eight hours after the first dose he develops fever, chills, myalgias, and headache. Which mechanism best explains this reaction?
5. A patient in the emergency department rapidly develops fever, flank pain, and dark urine while receiving an emergent transfusion of packed red blood cells that were improperly cross-matched. Which immunoglobulin class is most responsible for the intravascular hemolysis seen in this setting?
6. The therapeutic effect of this drug is achieved by potentiating the action of receptors that, when activated, primarily mediate an influx of which ion into the postsynaptic neuron?
7. This patient most likely has an inherited germline mutation in which of the following types of genes?
8. The accumulation of which substance is responsible for this constellation of findings?
9. This finding indicates that the nondisjunction event occurred during which of the following stages?
10. A protein purification scheme begins with ammonium-sulfate precipitation, followed by ion-exchange chromatography and ends with size-exclusion (gel-filtration) chromatography. During the final step, the protein of interest is found in the early fractions that elute soon after the void volume. Which property of the protein best explains this elution behavior?
11. Which of the following pharmacodynamic changes is the most likely cause of the decreased effectiveness of albuterol in this patient?
12. Using this size-exclusion chromatography technique, in which fractions would the 500 kDa protein complex be expected to appear?
13. This strain most likely has a mutation in the gene encoding which of the following enzymes?
14. A 40-year-old female has ptosis, ophthalmoplegia, and mild proximal weakness. Lactate is 3.4 mmol/L (0.5–2.2). Muscle biopsy shows ragged red fibers, indicating abnormal mitochondrial proliferation. Mitochondria generate ATP via oxidative phosphorylation; defects increase NADH and favor lactate formation. Which of the following laboratory findings is most indicative of mitochondrial dysfunction?
15. A 55-year-old fisherman presents with rapidly progressive cellulitis of the right lower leg following a puncture wound sustained while shucking oysters. Blood cultures grow curved, oxidase-positive, gram-negative rods that are halophilic. Which of the following selective media would most enhance recovery and preliminary identification of this pathogen from wound exudate?
16. Based on this result, what percentage of the variability in serum LDL cholesterol levels can be explained by the variability in daily saturated fat intake?
17. Which of the following host receptors is primarily responsible for recognizing LPS and initiating the innate immune response in this patient?
18. These findings are a predictable consequence of therapy with which of the following drugs?
19. The most likely source of this patient's bleeding is a pathologic lesion resulting from which of the following mechanisms?
20. What is the term for the co-existence of two or more mitochondrial DNA populations within a single individual?

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