USMLE Step 1 Microbiology Questions
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Free USMLE Step 1 Microbiology Questions

A 61-year-old female is brought to an urgent care clinic by her husband with altered mental status for 1 day. She was doing well until yesterday when she started to feel excessively drowsy. She had a renal transplant 18 months back. She is compliant with her immunosuppressive medications. On physical examination, her temperature is 39.4°C (103.0°F), blood pressure is 85/50 mm Hg, pulse 135/min and respirations are 24/min. Her Glasgow Coma Score is 10/15. There are few black skin lesions on the trunk with a necrotic center forming an eschar. Her laboratory investigation reveals the following:

Hemoglobin 14.2 g/dL
WBC count 3,700/µL
Neutrophils 22%
Lymphocytes 52%
Monocytes 17%
Eosinophils 5%
Basophils 4%
Platelets 179,000/µL
BUN 15 mg/dL
Creatinine 0.8 mg/dL
Blood cultures reveal Pseudomonas aeruginosa. Which of the following factors is the most likely responsible for her condition?
  1. Failure trimethoprim/sulfamethoxazole prophylactic therapy
  2. Failure of immunosuppressive therapy
  3. Decreased interleukin-2 levels
  4. DiGeorge Syndrome
  5. Decreased phagocytic cell count
A 23-year-old woman presents to the clinic for her first prenatal appointment with an unusual fatigue and pain in her perineum for the past 8 days. She does not have any significant past medical history and claims of unprotected intercourse only with her husband. She had an allergic reaction to amoxicillin two years ago. A vaginal speculum exam reveals a clean, ulcerated genital lesion, which is tender and non-exudative. No lymphadenopathy is detected. A rapid plasma reagin (RPR) test reveals a titer of 1:64 and the fluorescent treponemal antibody absorption (FTA- abs) is positive. What is the next best step in the management of this patient?
  1. Parenteral Ceftriaxone 1 g for 10 days
  2. Penicillin desensitization, then benzathine penicillin G 2.4 million units once intramuscular
  3. Tetracycline 500 mg orally 4 times daily for a week
  4. Doxycycline 100 mg twice daily for 14 days
  5. Hold treatment until delivery
A 33-year-old male living in the United States recently consumed a meal mostly consisting of raw shellfish that his girlfriend brought from her trip to Asia. Two days later he experienced a sudden onset of diarrhea and vomiting with severe abdominal cramps, while his girlfriend developed a mild diarrhea just several hours later. The diarrhea was profuse, looked like rice water, and had a pungent fishy odor. He soon started to experience muscle cramps and weakness, together with deep labored breathing pattern. In that moment they decided to call the ambulance and get to the hospital as soon as possible. According to his symptoms and bluish skin appearance, the attending physician hospitalized him, administered an intravenous infusion and sent a stool specimen to the clinical microbiology laboratory for analysis. The next day yellow bacterial colonies were observed on thiosulfate-citrate-bile salts-sucrose agar (as shown on the image). If you were the microbiologist on call, what kind of bacterial morphology would you expect to see during microscopic evaluation of a gram-stain slide made from those bacterial colonies?
  1. Comma-shaped rods
  2. Seagull-shaped rods
  3. Spiral-shaped rods
  4. Corkscrew-shaped rods
  5. Club-shaped rods
Microbiology USMLE Step 1 Practice Question

Image: by Microrao, JJMMC, Davangere, Karnataka, India, License: Public Domain

A 41-year-old woman presents with occasional dyspareunia and vaginal bleeding after having sex. She has a single sexual partner and uses oral contraception. She does not have a family history of any gynecological malignancy. She smokes a pack of cigarettes per day for 15 years and drinks alcohol moderately. She does not have HPV vaccination. Her blood pressure is 120/70 mm Hg, heart rate 71/min, respiratory rate 14/min, temperature 36.7°C (98.1℉). Speculum exam shows a centered round cervix centered to the middle with a reddish discoloration area with a size of approximately 1 x 2 cm. Bimanual examination reveals no apparent pathological changes. Papanicolaou test is shown in the exhibit. A gene coding for which of the following proteins is most likely to be mutated in the affected cells in this case?
  1. EGFR
  2. p53
  3. Myc
  4. Btk
  5. c-Src
Microbiology USMLE Step 1 Practice Question

Image: by National Cancer Institute, License: Public Domain

A 27-year-old dental radiographer came to the clinic with reddish lesions found on his palate, right lower and mid upper lip, as well as on one of his fingers. These lesions were accompanied by slight pain, and the patient had a mild fever 1 week before their appearance. Furthermore, the patient was touching the affected area repeatedly, which also resulted in the bleeding of the lesions. Two days prior to his visit he observed a small vesicular eruption on his right index finger, which merged and became cloudy on the day of the visit. These symptoms appeared for the first time in his life, and he did not report any drug usage. Once in the clinic, a Tzanck smear test which involved scrapings of the aforementioned lesions was performed by the attending physician immediately, and multinucleated epithelial giant cells were observed microscopically. According to the clinical presentation and histological finding, which viral infection should be suspected in this case?
  1. Herpangina
  2. Herpes simplex infection
  3. Hand-foot-and-mouth disease
  4. Varicella-zoster infection
  5. Measles

A 30-year-old woman comes to the clinic complaining of shaking chills, fever and headache for the last 3 days. She says that she recently returned from a trip to India where she was visiting family. There is no history of loss of consciousness or respiratory distress. Vitals noted are temperature of 38.9℃ (102.0℉), respiratory rate of 19/min, blood pressure of 120/80 mm Hg, rapid and thready pulse 94/min could be palpated. On general examination she looks pale and there is presence of jaundice in her upper sclerae. Laboratory findings are as follows:

Hct 30%
Total bilirubin  2.6 mg/dL
Direct bilirubin  0.3 mg/dL

Peripheral smear is shown in the exhibit. What is the most deadly complication of this condition?

  1. Heart block
  2. Facial paralysis
  3. Cerebral edema
  4. Aplastic crisis
  5. Rheumatoid arthritis
Microbiology USMLE Step 1 Practice Question

Image: by Ahmed et al., License: CC BY 2.0, cropped by Lecturio

A 51-year-old male visits his family physician with the complaint of heartburn which has been gradually increasing over the last 10 years. It gets worse with intake of spicy foods and improves with antacids. Past medical history is insignificant. He is a security guard and most often has long working hours at night. He admits to smoking 1.5 packs of cigarettes every day. Upper gastrointestinal endoscopy reveals a few gastric ulcers and regions of inflammation. A biopsy is taken which reveals gram-negative bacteria colonized on the surface of the regenerative epithelium of the stomach as shown in the micrograph below. Which of the following bacterial products is responsible for neutralizing the acidity of the stomach?
  1. β-lactamase
  2. Hyaluronidase
  3. Urease
  4. Streptokinase
  5. Prostaglandins
Microbiology USMLE Step 1 Practice Question

Image: by Ed Uthman, MD, License: CC-BY-SA 2.0

A group of friends went to fast-food restaurant for a college reunion party. Within the next few days, all of them complained of bloody diarrhea, fatigue, and confusion. Physical exam revealed neurological deficits. Laboratory tests show anemia, thrombocytopenia, and uremia. Lactate dehydrogenase (LDH) is raised while haptoglobin is decreased. Peripheral blood smears show fragmented RBCs. Coombs tests are negative. Which of the following is the responsible organism?
  1. Entamoeba histolytica
  2. E. Coli
  3. Campylobacter jejuni
  4. Shigella
  5. Salmonella
A group of scientists is studying various ways of bacterial reproduction. They find out that bacteria also exchange various materials via these reproductive processes. They are about to study one such method known as bacterial conjugation. Which of the following is an example of a bacterial action that occurs via the process they are studying?
  1. Two Pseudomonas aeruginosa bacteria sharing identical copies of a plasmid after sharing DNA through a sex pili
  2. Helicobacter pylori producing a prophage encoded toxin
  3. A strain of MRSA acquiring the gene of capsulation from another encapsulated strain via DNA extraction
  4. A multidrug resistance Shigella species passing resistance factor R to Streptococcus species
  5. Biomarkers on an E. coli read F+  and it cannot transmit the gentamycin resistance to other strains of the similar species
A 14-year-old girl presents to the clinic with the complaint of fever, headache, and muscle ache for the last two days. She also complains of malaise and pain in her joints. She just returned from a camping trip to Delaware. Her past medical history is insignificant. She denies any recent contact with a sick person. Her temperature is 101.0° F (38.3° C), pulse is 87/min, respirations are 17/min and blood pressure is 120/78 mm Hg. On physical examination, there is a 3-inch, red, erythematous, round rash with central clearing on the right leg as shown in the photograph. Antibodies against Proteus vulgaris OX-19 are absent. Which of the following is the most likely cause of her symptoms?
  1. Measles virus
  2. Borrelia burgdorferi 
  3. Rickettsia rickettsii
  4. Chlamydia trachomatis
  5. Dermatophytosis
Microbiology USMLE Step 1 Practice Question

Image: by Hannah Garrison, License: CC BY-SA 2.5

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