Free Internal Medicine Common Infectious Diseases Questions and Answers — Questions and Answers
Question 1: A 72-year-old male recently treated with clindamycin presents with 5-6 watery, non-bloody stools per day for three days. He has lower abdominal cramping but no fever. His WBC count is 10,500/μL and serum creatinine is 1.0 mg/dL. A stool test is positive for Clostridioides difficile toxin. According to the most recent IDSA guidelines, which of the following is the most appropriate initial treatment for this patient's non-severe C. difficile infection?
- Oral Metronidazole 500 mg three times daily for 10 days
- Oral Vancomycin 125 mg four times daily for 10 days
- Oral Fidaxomicin 200 mg twice daily for 10 days (Correct answer)
- Oral Rifaximin 400 mg three times daily for 10 days
Correct answer: Oral Fidaxomicin 200 mg twice daily for 10 days
The 2021 IDSA guidelines recommend fidaxomicin as the preferred therapy for an initial episode of non-severe C. difficile infection based on superior sustained response rates. Oral vancomycin is an acceptable alternative. Metronidazole is no longer recommended for initial treatment of even mild-to-moderate CDI due to inferior efficacy compared to vancomycin and fidaxomicin.
Question 2: A 28-year-old healthy, non-pregnant female presents with a 3-day history of dysuria, urinary frequency, and urgency. She has no fever or flank pain. A urine dipstick is positive for nitrites and leukocyte esterase. Assuming no drug allergies and local E. coli resistance rates to trimethoprim-sulfamethoxazole are known to be >20%, which of the following is the most appropriate first-line oral antibiotic?
- Nitrofurantoin (Correct answer)
- Ciprofloxacin
- Amoxicillin-clavulanate
- Cephalexin
Correct answer: Nitrofurantoin
For uncomplicated cystitis in regions where E. coli resistance to trimethoprim-sulfamethoxazole is high (>20%), nitrofurantoin is a recommended first-line agent according to IDSA guidelines. Fluoroquinolones like ciprofloxacin should be reserved for more serious infections to avoid promoting resistance and due to potential side effects. Beta-lactams like amoxicillin-clavulanate and cephalexin are considered alternative agents but generally have lower efficacy for cystitis.
Question 3: A 55-year-old male with type 2 diabetes presents with a 2-day history of a warm, tender, erythematous rash on his left lower leg. The area is non-fluctuant and there is no purulent drainage or abscess. He is afebrile and hemodynamically stable. What is the most appropriate initial oral antibiotic therapy for this patient's non-purulent cellulitis?
- Trimethoprim-sulfamethoxazole
- Doxycycline
- Clindamycin
- Cephalexin (Correct answer)
Correct answer: Cephalexin
This patient presents with non-purulent cellulitis, which is most commonly caused by beta-hemolytic streptococci. Therefore, empiric treatment should primarily target this organism. Cephalexin is a first-generation cephalosporin with excellent activity against both streptococci and methicillin-susceptible Staphylococcus aureus (MSSA). Trimethoprim-sulfamethoxazole and doxycycline have poor activity against streptococci and are typically reserved for purulent infections where MRSA is a concern.
Question 4: A 45-year-old man presents with a 36-hour history of fever, myalgias, headache, and a non-productive cough during peak influenza season. A rapid influenza antigen test is positive. For an otherwise healthy outpatient, antiviral treatment with a neuraminidase inhibitor is most effective when initiated within what timeframe from symptom onset?
- Within 24 hours
- Within 48 hours (Correct answer)
- Within 72 hours
- At any time as long as the patient is symptomatic
Correct answer: Within 48 hours
Clinical trials have demonstrated that neuraminidase inhibitors, such as oseltamivir, provide the greatest clinical benefit by reducing the duration and severity of symptoms when they are initiated within 48 hours of illness onset in outpatients with uncomplicated influenza. Benefit may be seen when started later in hospitalized patients or those with severe disease, but the 48-hour window is key for maximal efficacy in the typical outpatient setting.
Question 5: A 35-year-old healthcare worker has a newly positive tuberculin skin test (12 mm induration) as part of an annual screening. A chest X-ray is normal, and she denies any symptoms such as cough, fever, or weight loss. Which of the following is a preferred short-course regimen for the treatment of latent tuberculosis infection (LTBI) in this patient?
- Isoniazid and rifapentine weekly for 3 months (Correct answer)
- Isoniazid daily for 9 months
- Rifampin daily for 6 months
- Ethambutol and pyrazinamide daily for 2 months
Correct answer: Isoniazid and rifapentine weekly for 3 months
The CDC and ATS currently recommend several rifamycin-based, short-course regimens for the treatment of LTBI due to their efficacy and higher rates of treatment completion. The 3-month regimen of once-weekly isoniazid plus rifapentine (3HP) is a preferred option. Other preferred short-course options include 4 months of daily rifampin (4R) or 3 months of daily isoniazid and rifampin (3HR). While 9 months of isoniazid is still an acceptable alternative, shorter courses are now favored. Ethambutol and pyrazinamide are used for active TB, not latent infection.
Question 6: A 40-year-old man with a history of intravenous drug use presents with fever and malaise. Blood cultures are drawn, and an echocardiogram is ordered. According to the modified Duke criteria for infective endocarditis, which of the following is considered a major criterion for diagnosis?
- Fever > 38.0°C (100.4°F)
- Vascular phenomena, such as Janeway lesions
- Immunologic phenomena, such as Osler nodes
- Positive blood culture with a typical microorganism (Correct answer)
Correct answer: Positive blood culture with a typical microorganism
The modified Duke criteria categorize clinical findings into major and minor criteria. The two major criteria are: (1) positive blood cultures with a microorganism typical for causing infective endocarditis (e.g., S. aureus, Viridans streptococci, Enterococcus), and (2) evidence of endocardial involvement, such as a new valvular regurgitation or a positive echocardiogram (vegetation, abscess). Fever, vascular phenomena (Janeway lesions), and immunologic phenomena (Osler nodes) are all considered minor criteria.
