DMF Week 9 SAQ

1

Which one or more of the techniques listed below would be useful in the diagnosis of the cause of gastroenteritis in several members of a family?

A - Culture of faeces on a range of microbiological media

B - Culture of faeces in cell cultures for the detection of viruses

C - Microscopic examination of a wet preparation of faeces

D - Gram stain of faeces


  A, B, and C only are correct

  A and C only are correct

  B and D only are correct

  D only is correct

  All are correct


1 is incorrect. 2 is correct: Faeces should be cultured on a wide range of media in order to identify any of the wide range of bacterial pathogens that may cause gastroenteritis. Microscopic examination of a wet preparation of faeces, typically after concentration of the sample and often with the addition of iodine, will reveal possible parasites which may be involved. 3 is incorrect: The viruses that cause gastroenteritis cannot readily be grown in cell culture, and their presence in faeces is usually detected by antigen detection assays. 4 is incorrect: In the majority of cases of gastroenteritis, Gram staining cannot distinguish between bacteria that are part of the normal microbiota of the GIT, and those which are pathogens. 5 is incorrect.

2

Which one or more of the following statements describe the mechanisms by which pathogenic E. coli cause diarrhoea?

A - The release of heat-stable enterotoxin

B - The invasion of colonic epithelium

C - The production of attachment-effacement lesions

D - The production and release of cytotonic toxins


  A, B, and C only are correct

  A and C only are correct

  B and D only are correct

  D only is correct

  All are correct


These statements describe mechanisms used by different pathogenic E. coli (ETEC, EIEC, EPEC, ETEC) to cause gastrointestinal disease.

3

Which one or more of the following statements about parasite lifecycles is/are correct?

A - Parasites always reach sexual maturity in the reservoir host.

B - Definitive hosts of parasites differ from intermediate hosts because, although development occurs in both, sexual maturity only occurs in the definitive host.

C - Parasites only replicate in the reservoir host.

D - In paratenic hosts, parasites remain infective but do not undergo development.


  A, B, and C only are correct

  A and C only are correct

  B and D only are correct

  D only is correct

  All are correct


Statement B describes the definition of a definitive host, and statement D the definition of a paratenic host.

4

Which one of the following statements about malaria caused by Plasmodium falciparum is correct?


  Plasmodium falciparum is known to relapse due to the persistence of dormant parasites within the liver.

  Plasmodium falciparum infection is associated with a high level of red blood cell parasitemia due to the ability of the protozoan to infect erythrocytes in all stages of their development.

  Plasmodium falciparum malaria is unlike malaria caused by Plasmodium vivax because P. falciparum does not cause the syndrome known as cerebral malaria.

  Plasmodium falciparum relies on transmission to humans from primates which act as reservoir hosts.

  Plasmodium falciparum can always be diagnosed by detection of parasites in blood films.


1 is incorrect: In contrast to P. vivax, this species does not have a persistent liver stage associated with its life cycle. 2 is correct: This high level of parasitemia contributes to the severity of disease associated with this organism. 3 is incorrect: P. falciparum is the cause of the syndrome known as cerebral malaria. 4 is incorrect: Mosquitos (typically the Anopheles mosquito) are the only reservoir hosts for the species of Plasmodia that cause disease in humans. 5 is incorrect: Screening blood films for the presence of malaria parasites whilst being the “gold standard” for diagnosis, is relatively insensitive, and at least 3 blood samples should be examined.

5

A 45 yr old non-Caucasian male was referred to the emergency department of the local hospital with malaise, headache and myalgia for the past week. He had no relevant medical history, and was not taking any medications. On examination, he had a temperature of 38oC. Physical examination was normal, however his pulse rate was slow. He had visited his brother in India, and had returned one week ago. He took prophylaxis for malaria, and had been vaccinated against hepatitis A and B, cholera, Japanese encephalitis and polio.

Which one or more of the specimens below is appropriate for the diagnosis of his condition?

A - Three sets of blood cultures

B - Blood for serology

C - Thick and thin blood films

D - A faeces sample


  A, B, and C only are correct

  A and C only are correct

  B and D only are correct

  D only is correct

  All are correct


This patient’s symptoms are consistent with a wide range of infections which could be associated with certain bacteria (eg Salm.Typhi) for which culture of blood and faeces as well as serology would be appropriate, a range of viruses (eg dengue virus), for which serology would be appropriate, and parasites (eg Plasmodia) for which examination of blood films would be appropriate.

6

No parasitic forms were seen in any of the specimens collected from the patient above. A bacterium considered to be the cause of his infection was grown from appropriate specimens. The organism was a Gram negative bacillus, which grew on HBA (horse blood agar) in aerobic and anaerobic conditions. The organism produced a colony of 2 mm diameter on MacConkey agar, and was a lactose non-fermenter.

Which one of the organisms listed below is the most likely cause of the patient’s condition?


  Salmonella Typhi

  Salmonella Typhimurium

  Campylobacter jejuni

  Neisseria meningitidis

  Pseudomonas aeruginosa


1 is correct: Salmonella Typhi produces disease consistent with the patient’s symptoms, and the characteristics of the organism cultured from the patient’s specimen (blood, and perhaps faeces) are also consistent with this diagnosis. 2 is incorrect: Infection with Salmonella Typhimurium is unlikely to produce these symptoms. 3 is incorrect: Infection with Campylobacter jejuni is unlikely to produce these symptoms, and the organism does not have these characteristics. 4 is incorrect: Infection with Neisseria meningitidis is unlikely to produce these symptoms, and the organism does not have these characteristics. 5 is incorrect: Infection with Pseudomonas aeruginosa is unlikely to produce these symptoms, and the organism does not have these characteristics.

7

Which one of the antimicrobial agents listed below would be most appropriate for this patient?


  Metronidazole

  Penicillin

  Ampicillin

  Ciprofloxacin

  Gentamycin