10 USMLE Infectious Disease Questions with Answers and Explanations (Set 2)

These 10 USMLE infectious disease practice questions (Set 2) cover atypical pneumonia, sexually transmitted infections, congenital and foodborne parasites, tuberculosis drugs, fungal meningitis in AIDS and antibiotic-associated diarrhea. 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 40 year old woman with a history of type 1 diabetes presents to the clinic for left ear pain and fullness. She also complains of decrease of hearing on the affected side. The patient didn’t measure her blood sugar for 6 months although she has glucometer at home. Her temperature is 99.8 F. Examination reveals tenderness and erythema of the left tragus extending to the left side of the face. It also reveals edema of the left external auditory meatus with inflammatory debris. The tympanic membrane appears intact and mobile. Her blood glucose is 340 mg/dL. What is the most appropriate next step in management?

A- Antibiotic ear drops and follow up in three days
B- Empiric oral antibiotic therapy and follow up in one week
C- Hospitalization for intravenous antibiotics and control of diabetes
D- Initiation of left myringotomy and induction of topical treatment

This patient presents with the classic signs and symptoms of extensive otitis externa. There is an evidence of tendency to progress to facial cellulitis. In cases of diabetic and immunocompromised patients, aggressive interaction should be done. Hospitalization and intravenous antibiotics are the appropriate next step of action in this case. Antibiotics ear drops is only used if the patient is not diabetic. Initiation of left myringotomy is not used in otitis externa.
The correct answer is C

 

Question 2. A 22-year-old man visits the clinic for evaluation of fever, headache and dry cough for 7 days. His medical history is unremarkable and he takes no medications. He had no previous similar episodes. On examination, temperature is 101.3 F, pulse is 88/min. and blood pressure is 100/60 mmHg. On auscultation, his lungs are clear bilaterally. X-ray shows diffuse infiltration in the lower lobes of both lungs. What is the most likely causative organism?

A- Streptococci
B- Mycobacteria
C- Mycoplasma
D- Fungi
E- Rickettsia

Mycoplasma pneumonia usually has an atypical presentation and characterized by influenza-like symptoms as fever, headache, malaise and dry cough. Diffuse infiltration in the lower lobes bilaterally differentiates Mycoplasma from the other listed organisms.
The correct answer is C

 

Question 3. A 42-year-old man complains of rash and itching. He has no fever, headache or myalgia. On examination, there is maculopapular rash on the abdomen, palms of the hands and soles of the feet. There are also hypertrophic wart-like lesions around the anal area. Cervical, occipital and inguinal lymph nodes are enlarged. CBC shows marked increase in lymphocytes. What is the most appropriate laboratory test to confirm his diagnosis?

A- Nucleic acid amplification test for chlamydia
B- Serology for Human papilloma virus
C- VDRL for syphilis
D- Biopsy of the anal lesions
E- Stool and urine analysis

This patient most probably has secondary syphilis characterized by rash involving palms of the hands and soles of the feet. It is also characterized by condyloma lata, which are hypertrophic wart-like lesions and specific for syphilis. VDRL test is positive in all cases of secondary syphilis.
The correct answer is C

 

Question 4. A 31-year-old pregnant woman presents to her physician on her second trimester for a prenatal checkup. She works at a big farm. She has no concerns except for some mild swellings on her neck. On physical examination, her vital signs are within normal limits. She has mild cervical lymphadenopathy. She is prescribed an oral antibiotic but she is noncompliant. Her child is delivered with hydrocephalus and intracranial calcifications. Which organism is the most likely responsible?

A- Toxoplasma gondii
B- Treponema pallidum
C- Trypanosoma cruzi
D- Togavirus
E- Herpes simplex virus

Toxoplasma gondii is the most likely responsible for this condition. Her age, occupation, physical examination and typical signs of hydrocephalus and intracranial calcifications in the fetus are the clues to answer this question.
The correct answer is A

 

Question 5. A 26-year-old man complains of cough, night fever, loss of weight, anorexia and night sweats for six weeks.. Physical examination reveals vital signs within normal limits except for slight fever. Chest x-ray shows a small cavity in the right middle lobe and enlarged bronchopulmonary lymph nodes. Blood analysis reveals lymphocytosis and his sputum is positive for acid-fast bacilli. A treatment plan is discussed with the patient and he starts taking his medications. Which of these drugs acts by inhibition of formation of mycolic acid?

A- Ethambutol
B- Isoniazid
C- Pyrazinamide
D- Rifampin
E- Streptomycin

This is a case of primary tuberculosis (TB) confirmed by positive sputum analysis. Tuberculosis can be primary or secondary. Primary TB represents by Ghon’s complex which consists of a subpleural lesion between the upper and lower lobes and enlarged pulmonary lymph nodes while secondary TB is located in the apex of one or both lungs. Primary TB is characterized by cough, night fever, evening sweats, and anorexia and weight loss as seen in this patient. Anti-tuberculous drugs contain all the drugs listed in the question. Isoniazid is the only drug on the list that acts by inhibition of mycolic acid in the outer layer of mycobacteria. It is a synthetic analogue of Vitamin B6 (pyridoxine). Ethambutol and pyrazinamide have unknown mechanism. Rifampin inhibits bacterial DNA-dependent RNA polymerase leading to inhibition of RNA synthesis. Streptomycin is aminoglycoside inhibits protein synthesis by binding to 30S ribosomal unit.
The correct answer is B

 

Question 6. A 35-year-old man comes to the clinic complaining of myalgias and fever. Two weeks ago, he had experienced a brief, self-limited spell of diarrhea after going to a barbecue party. After that, there were no more symptoms and the patient remained asymptomatic. Yesterday evening, he developed fever of 103 F (39.4oC), severe muscle pain, and conjunctivitis. There is also generalized muscular soreness along with a diffuse maculopapular rash. Several minute hemorrhages can be seen under his fingernails. Complete blood count (CBC) performed at the time of admission shows white blood cell count of 15,000/mm3 with 30% eosinophils. Which one of these developmental forms is responsible for his infection?

A- Ovum
B- Cyst
C- Encysted larvae
D- Rhabditiform larvae

This patient is likely suffering from being infected by encysted larvae, also known as trichinosis. Infection occurs when humans eat undercooked pork products. The common symptoms of infection include vomiting, sweating, diarrhea, facial edema, fever, intense muscle pain, low blood pressure, weakening of pulse, difficulty breathing and heart damage. In severe cases, the condition may eventually lead to death from heart failure, kidney malfunction, or respiratory complications.
The correct answer is C

 

Question 7. A 22-year-old female with a history of acquired immunodeficiency syndrome (AIDS) develops low grade fever, headache and blurred vision. Cerebrospinal fluid (CSF) testing for Cryptococcal antigen is positive. Which of these drugs can be given orally to treat Cryptococcal meningitis?

A- Amphotericin B
B- Caspofungin
C- Fluconazole
D- Metronidazole

This is a tricky question. The ideal regimen for treating cryptococcal meningitis is IV amphotericin B followed by ORAL fluconazole. The question asks for orally drug otherwise, amphotericin would be the best answer. Caspofungin is one of the echinocandins which is used in candidiasis and is only available in IV preparations. Metronidazole is a distraction as it is not effective against Cryptococcus but it is indicated in cases of anaerobic bacterial infections.
The correct answer is C

 

Question 8. A 24-year-old man presents to the clinic with a complaint of grey-colored penile discharge and burning micturition for 2 days. His medical, surgical and family histories are unremarkable. Examination reveals a purulent discharge that can be seen by squeezing his penis. Penile swab shows Gram-negative diplococci within neutrophils. What is the most appropriate treatment?

A- Oral doxycycline alone
B- Oral azithromycin alone
C- IM ceftriaxone plus oral doxycycline
D- IV penicillin
E- IM ampicillin

This patient has gonococcal urethritis (Gram-negative intracellular diplococci). The treatment is a single dose of IM ceftriaxone (500 mg). Because chlamydia often occurs with gonorrhea, oral doxycycline for 7 days is added unless chlamydia has been ruled out. Penicillins are no longer used because of resistance, and doxycycline or azithromycin alone does not reliably treat gonorrhea.
The correct answer is C

 

Question 9. A 60-year-old man visits the clinic due to profuse watery diarrhea and left lower abdominal colicky pain for the last 6 hours. Upon taking history, the patient claims that he is taking broad spectrum antibiotics for acute bronchitis. Which is the most likely causative organism for the patient’s new symptoms?

A- Clostridium botulinum
B- Clostridium butyricum
C- Clostridium difficile
D- Clostridium histolyticum
E- Clostridium Septicum

Antibiotic-associated diarrhea is most likely caused by clostridium difficile. The relation of this organism to the use of certain antibiotics is not fully understood yet. Treatment of this condition is oral vancomycin. Other types of clostridia mentioned in the choices cause different pathological conditions i.e. botulism, general sepsis, etc.
The correct answer is C

 

Question 10. A 33-year-old butcher comes to the clinic because he developed eye problems. On taking history, the patient states that he eats raw hamburgers on a regular basis. The physician orders Sabin-Feldman dye test and the results come back positive. Which is the most likely cause of this patient’s condition?

A- Giardiasis
B- Schistosomiasis
C- Trichinosis
D- Toxoplasmosis

This is most likely a case of toxoplasmosis, even though all the above-listed conditions have significant clinical and epidemiologic features. Toxoplasmosis is a self-limiting and mild condition that occurs if the patient ingests Toxoplasma tissue cysts in undercooked meat or oocysts from cat feces. Toxoplasmosis is generally associated with Chorioretinitis, an inflammation of choroid and retina of the eye, thereby making toxoplasmosis the most likely cause of this patient’s condition.
The correct answer is D

 

High-yield points from this set

  • Ear pain with debris in a poorly controlled diabetic spreading to the face: admit for IV anti-pseudomonal antibiotics.
  • Dry cough, headache and patchy bilateral infiltrates in a young adult = Mycoplasma pneumonia.
  • Rash on palms and soles with condylomata lata = secondary syphilis; screen with VDRL/RPR.
  • Hydrocephalus and intracranial calcifications in a newborn = congenital toxoplasmosis.
  • Isoniazid inhibits mycolic acid synthesis.
  • Fever, myalgia, periorbital swelling, splinter hemorrhages and eosinophilia after undercooked pork = trichinosis.
  • Cryptococcal meningitis: IV amphotericin B plus flucytosine, then oral fluconazole.
  • Gonorrhea: IM ceftriaxone; add doxycycline if chlamydia is not excluded.
  • Watery diarrhea during or after antibiotics = Clostridioides difficile.
  • Chorioretinitis after eating raw meat = toxoplasmosis.

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