Metabolism & Bioenergetics USMLE Step 1 Practice Question
A 25-year-old woman with a positive urine pregnancy test presents with lower abdominal pain and vaginal bleeding at 7 weeks gestation. Pelvic ultrasound reveals an empty intrauterine cavity with free pelvic fluid and a 3-cm adnexal mass. Serum β-hCG is 1,850 mIU/mL. The patient is diagnosed with an ectopic pregnancy and treated with methotrexate 50 mg/m² intramuscularly. Which of the following metabolic changes occurs as a direct consequence of methotrexate's mechanism of action?
Answer choices
- ADecreased dihydrofolate accumulation due to increased dihydrofolate reductase activity
- BIncreased thymidine monophosphate synthesis from deoxyuridine monophosphate
- CAccumulation of deoxyuridine monophosphate with concurrent depletion of deoxythymidine monophosphateCorrect answer
- DIncreased tetrahydrofolate polyglutamate formation and enhanced nucleotide synthesis
- EDecreased phosphoribosyl pyrophosphate availability for purine synthesis
- FIncreased guanylate synthesis compensating for impaired pyrimidine synthesis
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