10 USMLE Infectious Disease Questions with Answers and Explanations (Set 1)

These 10 USMLE infectious disease practice questions cover classic microbiology and clinical infection cases that repeat on Step 1 and Step 2 CK: tetanus prophylaxis, Lyme disease, measles, malignant otitis externa, epiglottitis, adenovirus, endocarditis prophylaxis, histoplasmosis, croup and cerebral malaria. Each question includes the correct answer and an explanation.

Written by Dr. Ray Makar, MD, author of the Repeated USMLE Questions eBook series.

Prefer video? Watch daily USMLE case questions on MedFocus by Dr. Ray on YouTube.

How to use this page: read the question, pick your answer, then tap Show Answer and Explanation.

Questions

Question 1. A 6-year-old girl presents to the emergency room with her mother for a deep wound on her right forearm. Her mother tells you that her daughter was playing at school today and wounded herself when she fell on a big rock. When you ask her about her vaccination, she tells you that her daughter had all her vaccinations before the school started this year. What is the most appropriate therapy for this girl?

A- Tetanus toxoid
B- Tetanus antitoxin
C- Both tetanus toxoid and antitoxin
D- Neither tetanus toxoid or antitoxin

This girl already received her vaccination before the school started this year. For this reason, there is no vaccination therapy needed for this girl at this time.
The correct answer is D

 

Question 2. A 29-year-old man presents to his physician complaining of fatigue and lethargy for the past 3 days after he came back from a hiking trip in Northern California. During the trip, he noticed a rash on his left arm. The rash was smaller but now it’s expanding in size. On examination, the rash looks round with central clearing. What is the most likely organism involved in this condition?

A- Meningococci
B- Staphylococcus aureus
C- Borrelia burgdorferi
D- Rickettsia prowazekii
E- Treponema pallidum

This is most likely the characteristic rash of Lyme disease. It is caused by borrelia burgdorferi infection transmitted by Ixodes tick. Other microorganisms don’t show this particular rash.
The correct answer is C

 

Question 3. A 6-year-old boy presents with fever, malaise, cough, running nose and conjunctivitis. His past medical history indicates that he did not receive any immunizations due to his parents’ religious beliefs. On examination, his temperature is 102.7 F and there are white spots on a red base close to his second molar teeth. He also has a non-pruritic, maculopapular rash on his face, behind his ears, and spreading to his chest. What is the most likely diagnosis?

A- Chickenpox
B- German measles
C- Measles
D- Mumps
E- Smallpox

Koplik spots in the mouth are diagnostic of measles. They are white spots on a red base and found close to second molars.
The correct answer is C

 

Question 4. A 68-year-old man with a long history of poorly controlled diabetes presents to his physician complaining of pain and drainage in his left ear. His vital signs are within normal limits. The pinna of his left ear is tender. The external meatus is swollen and covered with purulent drainage. What is the most likely organism responsible for his condition?

A- Candida albicans
B- Haemophilus influenzae
C- Pseudomonas aeruginosa
D- Moraxella catarrhalis
E- Streptococcus pneumoniae

The patient’s symptoms suggest a case of malignant otitis externa (MOE), also known as necrotizing otitis externa. This condition is often seen in elderly patients, especially those with poorly controlled diabetes. The typical presentation includes severe ear pain, swelling, purulent discharge, and sometimes granulation tissue in the ear canal.
The most likely organism responsible for malignant otitis externa is Pseudomonas aeruginosa.
The correct answer is C

 

Question 5. A 34-year-old man with a one-week history of sore throat presents today with fever, hoarseness, difficulty breathing, and drooling. On physical examination, his temperature is 102.3f and he has inspiratory stridor. What is the most preferred next action for this patient?

A- Admit to the intensive care unit
B- Prescribe broad-spectrum antibiotic
C- Throat swab and culture
D- Serology for infectious mononucleosis
E- Ultrasonography of the neck

This patient most likely has acute epiglottitis with impending airway obstruction. This is an emergency. He needs to be admitted instantly to intensive care unit. H. influenzae is the most common cause of this case. Other options are the steps should be done later after admission to ICU.
The correct answer is A

 

Question 6. Three boys are brought to the urgent care clinic complaining of fever, sore throat and itchy eyes. On examination, their temperature range from 101.2 F to 102.7 F. Their conjunctivas are red and inflamed. They are a part of a group of five kids from elementary school camp out on the lake. Which of these is the most likely causative organism of this illness?

A- Adenovirus
B- Coxsackie virus
C- Echovirus
D- Measles
E- Rubella

Adenovirus is the most common causative organism of common cold and pink eye in children. It spreads fast on schools and camps. Other viruses rarely cause these combined symptoms.
The correct answer is A

 

Question 7. A 29-year-old woman with a history of rheumatic heart disease and prosthetic aortic valve presents to her physician for consultation. She is referred by her dentist for prophylaxis before she undergoes an oral surgery next month. She has no history of bacterial endocarditis. She is allergic to penicillin. Which of the following is the prophylactic drug of choice for this patient?

A- Ampicillin
B- Amoxicillin
C- Azithromycin
D- Gentamicin
E- Vancomycin

Amoxicillin is the usual drug of choice for dental prophylaxis in high-risk patients, such as those with a prosthetic heart valve. Because this patient is allergic to penicillin, azithromycin or clarithromycin (or doxycycline) is used instead. Clindamycin is no longer recommended for this purpose.
The correct answer is C

 

Question 8. Two sisters, aged 40 and 45 years old, are brought to the emergency room presenting with severely dramatic pneumonia. When the lungs are examined, doctors are able to easily hear rales. The chest x-rays reveal that both sisters have bilateral and diffuse infiltrates. On taking down their history, it was observed that 2 weeks ago both sisters went hiking to a mountainous area where they explored a dusty cave. What would be the most likely causative agent of their condition?

A- Haemophilus influenzae
B- Blastomyces dermatitidis
C- Coxsackie A virus
D- Histoplasma capsulatum

The sisters are suffering from acute pulmonary histoplasmosis, which is a respiratory infection caused by inhalation of the spores of the Histoplasma capsulatum fungus. It is commonly found in bird and bat droppings, something which you would likely find in caves. You get sick upon breathing in the spores produced by the fungus.
The correct answer is D

 

Question 9. A 3-year-old girl is brought to the clinic by her parents because she woke up during the night with a severe cough. She recently developed an upper respiratory tract infection. Examination reveals noisy breathing on inspiration, flaring of the nostrils, marked retractions of the chest wall, and a severe barking cough. What is the most likely diagnosis?

A- Asthma
B- Bronchiolitis
C- Foreign body stuck in the main bronchus
D- Viral croup

The child is suffering from a viral croup. A barking cough is the most common symptom of a viral croup. A viral croup is also an infection of the upper respiratory tract, thus best-fitting the symptoms the child is having.
The correct answer is D

 

Question 10. A 45-year-old man is rushed to the emergency room having severe headaches and recurrent fever started 1 week ago, a week after he came back from a trip to Africa. Physical examination reveals high fever and splenomegaly. Brain imaging reveals signs of cerebral involvement. Which of these following parasites is likely causing the patient’s symptoms?

A- Babesia microti
B- Plasmodium falciparum
C- Plasmodium malariae
D- Plasmodium vivax

Plasmodium falciparum causes malaria. Spread by the Anopheles mosquito, P. falciparum infection is detected by a blood smear. The patient is suffering from cerebral malaria and the only species of Plasmodium that causes cerebral malaria is P. falciparum.
The correct answer is B

 

High-yield points from this set

  • A fully vaccinated child with a recent booster (less than 5 years) needs no tetanus toxoid or immune globulin, even for a dirty wound.
  • Expanding round rash with central clearing (erythema migrans) after hiking = Lyme disease (Borrelia burgdorferi).
  • Koplik spots (white spots on a red base near the molars) are diagnostic of measles.
  • Ear pain and discharge in an elderly diabetic = malignant otitis externa, usually Pseudomonas aeruginosa.
  • Drooling, stridor and fever = epiglottitis; secure the airway in the ICU first.
  • Fever, sore throat and conjunctivitis in a group of children = adenovirus (pharyngoconjunctival fever).
  • Penicillin-allergic patient needing dental prophylaxis: azithromycin, clarithromycin or doxycycline.
  • Pneumonia after exploring a cave with bat or bird droppings = histoplasmosis.
  • Barking cough with inspiratory stridor in a young child = viral croup (parainfluenza).
  • Cerebral malaria is caused by Plasmodium falciparum.

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