Weekly Year 2: Microbiology Exam - July 10, 2026 (Section...

Question 1

An outbreak of sepsis caused by Staphylococcus aureus has occurred in the newborn nursery. What is the most likely source of the organism?

Accepted Answer

The nasal passages of healthcare workers — Staphylococcus aureus is a common commensal organism found in the anterior nares of humans. Healthcare workers can easily transfer it to vulnerable infants, especially in a nursery setting.

Question 2

Each of the following agents is a recognized cause of diarrhea except:

Accepted Answer

Lactobacillus acidophilus — Lactobacillus acidophilus is a probiotic bacterium that is beneficial for gut health and is not typically associated with causing diarrhea. The other options are well-known causes of bacterial gastroenteritis or colitis.

Question 3

Bacterial identification by serological tests is based on specific antigens. Which component is least likely to contain useful antigens?

Accepted Answer

Ribosomes — Ribosomes are intracellular components involved in protein synthesis and are not typically exposed on the bacterial surface to elicit an antibody response for serological identification. Capsules, cell walls, and flagella are all external structures that contain immunogenic antigens.

Question 4

Which Pasteurella species has been associated with female genital tract and newborn infections?

Accepted Answer

Pasteurella multocida — Pasteurella multocida is commonly found in the oropharynx of domestic animals, particularly cats and dogs, and can be transmitted to humans, leading to a variety of infections including those of the genital tract and neonates.

Question 5

Each statement about streptococci classification is correct except:

Accepted Answer

Group A Streptococcus is typically identified by the bacitracin susceptibility test. — Group A Streptococcus (Streptococcus pyogenes) is typically identified by the bacitracin disc test, where it is susceptible. While this test helps in presumptive identification, it's not the sole basis of its classification. Lancefield grouping is indeed based on C carbohydrate antigens, alpha-hemolysis involves partial red blood cell lysis, and beta-hemolysis is divided into groups based on antigenic properties.

Question 6

Staphylococcal scalded skin syndrome is caused by exotoxin produced by:

Accepted Answer

Staphylococcus aureus — Staphylococcal scalded skin syndrome (SSSS) is caused by exfoliative toxins (epidermolytic toxins) produced by specific strains of Staphylococcus aureus.

Question 7

A pregnant woman with previous syphilis treatment shows: RPR non-reactive, TP-PA reactive. Which statement is most correct?

Accepted Answer

She has a past syphilis infection that has been successfully treated. — A non-reactive RPR (Rapid Plasma Reagin) test typically indicates the absence of active infection or successful treatment. A reactive TP-PA (T. pallidum particle agglutination) test confirms exposure to Treponema pallidum and can remain reactive for life, indicating past infection.

Question 8

Which bacteria has the lowest 50% infective dose (ID50)?

Accepted Answer

Shigella dysenteriae — Shigella species, particularly Shigella dysenteriae, have a very low ID50, often as low as 10-100 organisms, making them highly infectious even in small doses. Vibrio cholerae requires a larger dose, and while E. coli O157:H7 and Salmonella Typhi are significant pathogens, their ID50 is generally higher than Shigella.

Question 9

Which disease is best diagnosed by serologic means?

Accepted Answer

Viral hepatitis — Many viral infections, including viral hepatitis, are best diagnosed by detecting specific antibodies or antigens in the serum, as the causative virus may be difficult to culture or detect directly in clinical samples. Bacterial meningitis and UTIs are typically diagnosed by culture, and TB by culture or molecular methods, although serology plays a role in some specific TB diagnostics.

Question 10

A woman has non-bloody diarrhea for 14 hours. Which organism is least likely to cause this?

Accepted Answer

Campylobacter jejuni — Campylobacter jejuni typically causes bloody diarrhea (dysentery) due to its invasive nature. Norovirus and Staphylococcus aureus toxin are common causes of non-bloody, acute gastroenteritis. Vibrio cholerae causes profuse, watery (non-bloody) diarrhea.

Question 11

A woman develops gonococcal PID. What is the most common sequela?

Accepted Answer

All of the above — Pelvic Inflammatory Disease (PID) caused by Neisseria gonorrhoeae can lead to a range of serious long-term complications including infertility due to scarring of the fallopian tubes, chronic pelvic pain, and an increased risk of ectopic pregnancy.

Question 12

Mycobacterial survival after macrophage ingestion is attributed to:

Accepted Answer

Resistance to lysosomal enzymes — Mycobacteria possess unique mechanisms to resist the microbicidal activities of macrophages, including preventing the fusion of phagosomes with lysosomes and surviving the acidic and enzymatic environment within the phagolysosome. While catalase production can be a virulence factor, resistance to lysosomal enzymes is a more direct mechanism for survival within macrophages.

Question 13

Fish are second intermediate hosts of?

Accepted Answer

Clonorchis sinensis — Clonorchis sinensis (Chinese liver fluke) uses fish as second intermediate hosts. Humans become infected by consuming raw or undercooked infected fish. Schistosoma mansoni's intermediate hosts are snails. Taenia solium's intermediate host is pigs. Ascaris lumbricoides is a direct life cycle nematode.

Question 14

Which of the following secretes defensive secretions to cause irritation on human skin?

Accepted Answer

Certain beetles (e.g., blister beetles) — Certain beetles, like blister beetles, secrete a blistering agent called cantharidin, which causes significant skin irritation and blistering upon contact. Bed bugs, fleas, and lice cause irritation through their bites, but not by secreting a chemical irritant onto the skin.

Question 15

Holometabolous insects have?

Accepted Answer

Complete metamorphosis — Holometabolous insects undergo complete metamorphosis, which includes four distinct life stages: egg, larva, pupa, and adult. Examples include flies, mosquitoes, and butterflies.

Question 16

Which of the following transmits parasites for river blindness?

Accepted Answer

Blackflies — River blindness (Onchocerciasis) is transmitted by the bite of infected blackflies (Simuliidae).

Question 17

Which insecticide targets GABA in its action?

Accepted Answer

Fipronil — Fipronil is a phenylpyrazole insecticide that acts by blocking GABA-gated chloride channels in insects, leading to hyperexcitation of the nervous system and death. Organophosphates and carbamates inhibit acetylcholinesterase, pyrethroids affect sodium channels, and neonicotinoids target nicotinic acetylcholine receptors.

Question 18

Presence of triatomine bugs could predispose the population to?

Accepted Answer

Chagas disease — Triatomine bugs, also known as kissing bugs, are vectors for Trypanosoma cruzi, the parasite that causes Chagas disease.

Question 19

People living in forested areas like Arabuko Sokoke are at a risk of?

Accepted Answer

Leishmaniasis — Forested areas in certain regions, particularly around the Mediterranean and Middle East, are known habitats for sandflies, which transmit the parasites causing leishmaniasis. Lyme disease and RMSF are tick-borne, and while possible in forests, leishmaniasis is a stronger association with specific forested regions and sandfly vectors.

Question 20

Which of the following insects are commonly found in Latin America?

Accepted Answer

Triatomine bugs — Triatomine bugs (kissing bugs) are endemic to the Americas, particularly Central and South America, and are vectors of Chagas disease.

Question 21

Pruritus can be best diagnosed for?

Accepted Answer

Scabies — Scabies is a parasitic infestation of the skin caused by mites, and intense itching (pruritus), especially at night, is its hallmark symptom and a key diagnostic indicator.

Question 22

If you found lice eggs (nits) in someone's clothing, what species would they most likely be from?

Accepted Answer

Pediculus humanus humanus (body louse) — Body lice (Pediculus humanus humanus) commonly lay their eggs (nits) in the seams of clothing, particularly around the waistband and underarms. Head lice lay nits on hair shafts, and pubic lice attach to pubic hair.

Question 23

Which of the following is odd one out in terms of transmission?

Accepted Answer

Housefly — Mosquitoes, ticks, and fleas are all biological vectors, meaning the pathogen undergoes part of its life cycle within the vector. Houseflies are primarily mechanical vectors, transmitting pathogens on their external surfaces from contaminated sources to food, without the pathogen developing within the fly.

Question 24

Which one of the following is larviparous?

Accepted Answer

Housefly — Houseflies are larviparous, meaning they lay eggs from which larvae (maggots) hatch. Tsetse flies are pupiparous (give birth to pupae), and mosquitoes and fleas lay eggs that hatch into larvae.

Question 25

Which one of the following is a complication of streptococcus pharyngitis?

Accepted Answer

Rheumatic fever — Rheumatic fever and acute glomerulonephritis are two major non-suppurative complications of Group A Streptococcus (Streptococcus pyogenes) pharyngitis. Otitis media and sinusitis are suppurative complications.

Question 26

The presence of a pseudomembrane is a result of:

Accepted Answer

Toxin production by Corynebacterium diphtheriae — The characteristic thick, grayish pseudomembrane on the tonsils and pharynx in diphtheria is formed by the combination of bacteria (Corynebacterium diphtheriae), dead epithelial cells, fibrin, leukocytes, and erythrocytes, primarily induced by the diphtheria toxin.

Question 27

A paroxysm is a associated with :

Accepted Answer

Whooping cough — Paroxysms of coughing, characterized by severe, rapid, and repeated coughing fits followed by a characteristic 'whoop' sound upon inhalation, are the hallmark of pertussis (whooping cough), caused by Bordetella pertussis.

Question 28

The Mantoux test is used to identify:

Accepted Answer

All of the above — The Mantoux tuberculin skin test (TST) indicates that an individual has been infected with Mycobacterium tuberculosis. It can be used to detect both latent and active tuberculosis, and by definition, indicates past exposure to the bacteria.

Question 29

To treat patients who have ingested botulism toxin, large doses of must be administered:

Accepted Answer

Antitoxin — Botulism is caused by a neurotoxin produced by Clostridium botulinum. Treatment involves administering antitoxin to neutralize the circulating toxin before it can bind to nerve endings. Antibiotics are not effective against the toxin itself.

Question 30

The organism Clostridium perfringens multiplies in foods only under conditions:

Accepted Answer

Anaerobic — Clostridium perfringens is an obligate anaerobe, meaning it can only grow and multiply in the absence of oxygen. This is why it is a common cause of food poisoning when foods are prepared in large batches and kept warm for extended periods, creating anaerobic conditions.

Question 31

A spore-forming, motile rod that can cause foodborne illness in a diarrheal or emetic form is Bacillus:

Accepted Answer

cereus — Bacillus cereus is a spore-forming, motile bacterium known to cause two types of food poisoning: an emetic (vomiting) form, often associated with rice, and a diarrheal form, associated with other foods like meats and vegetables.

Question 32

Woolsorter disease applies to the form of :

Accepted Answer

Anthrax — Woolsorter's disease is a historical term for the inhalational form of anthrax, which was a significant occupational hazard for workers in the wool trade who handled infected animal fibers.

Question 33

Which one of the following describes the mode of action of tetanospasmin?

Accepted Answer

Inhibits inhibitory neurotransmitters at the neuromuscular junction. — Tetanospasmin, the neurotoxin produced by Clostridium tetani, blocks the release of inhibitory neurotransmitters (GABA and glycine) in the central nervous system, leading to uncontrolled muscle spasms (spastic paralysis).

Question 34

Salpingitis is associated with and can lead to :

Accepted Answer

Gonococcal infection; infertility — Salpingitis is inflammation of the fallopian tubes, most commonly caused by sexually transmitted infections like Neisseria gonorrhoeae. Untreated salpingitis can lead to scarring and adhesions, resulting in infertility, ectopic pregnancy, and chronic pelvic pain.

Question 35

Ophthalmia is associated with what two STDs?

Accepted Answer

Chlamydia trachomatis and Neisseria gonorrhoeae — Ophthalmia neonatorum, or conjunctivitis in newborns, can be caused by Chlamydia trachomatis and Neisseria gonorrhoeae acquired during passage through the birth canal. Both are sexually transmitted diseases.

Question 36

Toxin-generated S. aureus diseases resulting from contact include:

Accepted Answer

Impetigo and Folliculitis — Impetigo and Folliculitis are superficial skin infections caused by Staphylococcus aureus where toxins produced by the bacteria contribute to the pathogenesis and spread of the lesions through direct contact. Toxic Shock Syndrome and food poisoning are systemic or gastrointestinal effects of toxins. Pneumonia, endocarditis, osteomyelitis, and septic arthritis are invasive infections.

Weekly Year 2: Microbiology Exam - July 10, 2026 (Section...

Questions, Answers & Explanations