DMF Week 10 SAQ

1

A one year old, fully vaccinated child presents with pharyngitis, but no rhinitis.

Which one of the organisms listed below is the most likely cause of her condition?


  Rhinovirus

  Adenovirus

  Haemophilus influenzae type B

  Bordetella pertussis

  Streptococcus pyogenes


1 is incorrect: Whilst rhinoviruses are one of the most common causes of rhinitis and the common cold, pharyngitis is a mild symptom in their presentation. 2 is correct: Adenoviruses are common causes of pharyngitis, with or without rhinitis. 3 is incorrect: Although H. influenzae may colonise the throat and occasionally cause epiglottitis, this patient has been fully vaccinated against Haemophilus influenzae type B. 4 is incorrect: B. pertussis is a common cause of lower respiratory tract infection (laryngotracheitis, croup). The patient has also been vaccinated, which reduces the likelihood of infection. 5 is incorrect: Whilst S. pyogenes is the most common bacterial cause of pharyngitis, it is most commonly found in the 4 – 13yrs age group. Viruses are more common causes of pharyngitis.

2

Which one of the following is not commonly associated with Group A streptococcus infection?


  Pharyngitis

  Necrotising fasciitis

  Neonatal meningitis

  Septic arthritis

  Scarlet fever


1 is incorrect: Group A streptococci (Streptococcus pyogenes) are the most common bacterial cause of pharyngitis. 2 is incorrect: Group A streptococci are common causes of necrotising fasciitis. 3 is correct: Group A streptococci are not common causes of neonatal meningitis. 4 is incorrect: Group A streptococci are common causes of septic arthritis. 5 is incorrect: Group A streptococci are the cause of scarlet fever.

3

Which one of the statements about infection with Mycobacterium tuberculosis is correct?


  It is cured by a 7 day course of three drugs

  Infection with the organism commonly results in disease

  It progresses rapidly once the bacteria are established in the lung tissue

  It reactivates as antibody levels decline

  It can be detected by a simple skin test


1 is incorrect: Disease due to Mycobacterium tuberculosis requires administration of antibiotics for a prolonged period of time, typically 6 months for a “short course” of treatment. 2 is incorrect: Disease due to Mycobacterium tuberculosis is an rare outcome of infection. 3 is incorrect: Infection due to Mycobacterium tuberculosis in the lung is usually controlled by activated T cells, and remains latent, in most people for life. 4 is incorrect: Control of disease due to Mycobacterium tuberculosis does not appear to be mediated by antibodies, but by activated T cells. Should something affect the activity of these cells, reactivation tuberculosis may occur. 5 is correct: Infection with or exposure to antigens associated with Mycobacterium tuberculosis can be detected by the Mantoux (Tuberculin) skin test, which is a delayed type hypersensitivity skin test.

4

Which one of the following statements about the laboratory diagnosis of Mycobacterium tuberculosis is correct?


  The organism appears as a pink bacillus in a Ziehl-Neelsen stain

  The organism will grow after 4 days on Lowenstein-Jensen medium

  The organism is acid-fast because it has a large amount of peptidoglycan in its cell wall

  The organism appears as a purple bacillus in a Gram stain

  The Ziehl-Neelsen stain is the prefered laboratory technique for the diagnosis of tuberculosis


1 is correct: These are the “acid-fast bacilli” of M. tuberculosis. 2 is incorrect: M. tuberculosis takes at least 4 weeks to grow on Lowenstein-Jensen medium. 3 is incorrect: M. tuberculosis is acid-fast because it has a large amount of complex lipids, especially mycolic acids, which provide a waxy cell wall. 4 is incorrect: Although the organism has a peptidoglycan layer external to the cytoplasmic membrane, it cannot take up Gram stain reagents due to the thick outer layer of complex lipids. 5 is incorrect: Whilst the Ziehl-Neelsen stain provides a rapid presumptive diagnosis of tuberculosis, it is much less sensitive for the diagnosis of tuberculosis than culture. As well, culture is required to determine the species of the isolate and its antimicrobial susceptibilities.

5

A patient undergoing surgery for peritonitis due to perforated viscus was treated with i.v. ceftriaxone and metronidazole for surgical prophylaxis.

Which one of the following options best describes the rationale for this therapy?


  The combination is synergistic

  The combination will reduce the emergence of resistance

  This combination will reduce the individual toxicity of the antibitics

  This combination will target the range of organisms likely to be associated with a post operative infection at this site

  This combination is clinically determined to provide the best prophylaxis for all forms of surgery


1 is incorrect: This combination is not known to be synergistic. 2 is incorrect. 3 is incorrect: These antibiotics are relatively non-toxic. 4 is correct. 5 is incorrect: Antimicrobials used for sugical prophylaxis will vary depending on the surgical site and sources of microbial contamination.