Medical Bacteriology and Entomology | MCQ Quiz | OmpathStudy Kenya

Question 1

A 22-year-old woman who works in a plant nursery presents with a history of fever and cough for 2 months. Over this period she has lost 5 kg. Chest radiography shows bilateral upper lobe infiltrates with cavities. A stain of her sputum shows acid-fast bacilli. The likely means by which the patient acquired her infection is: A) Sexual activity

Accepted Answer

** Breathing aerosolized droplets containing the microorganism — The clinical presentation (chronic cough, weight loss, bilateral upper lobe cavities, acid-fast bacilli in sputum) is classic for pulmonary tuberculosis caused by Mycobacterium tuberculosis . TB is primarily transmitted through airborne droplets when infected individuals cough or sneeze. ---

Question 2

During a pandemic, 175 airline passengers flew from Lima, Peru, to Los Angeles. Lunch included crab salad eaten by two-thirds of passengers. Two passengers who stayed in Los Angeles developed severe watery diarrhea. The likely cause of the diarrhea is: A) Escherichia coli O157:H7

Accepted Answer

** Vibrio cholerae type O139** — The severe watery diarrhea following consumption of seafood (crab salad) from Peru during a pandemic outbreak is characteristic of cholera. Vibrio cholerae is commonly associated with contaminated seafood and causes profuse watery diarrhea. ---

Question 3

A 65-year-old woman with a long-term central venous catheter develops fever and multiple blood cultures positive for Staphylococcus epidermidis . A biofilm is thought to be present on the catheter. Which statement about such an infection is correct? A) The biofilm containing S. epidermidis is likely to wash off the catheter

Accepted Answer

** The complex molecular interactions within the biofilm make it difficult to provide effective antimicrobial therapy, and the catheter will likely need to be removed — Biofilms are notoriously difficult to treat with antibiotics because the extracellular matrix protects bacteria and reduces antibiotic penetration. Catheter removal is often necessary for cure. ---

Question 4

The first microorganism to satisfy Koch's postulates (in the late 19th century) was: A) Treponema pallidum

Accepted Answer

** Bacillus anthracis** — Robert Koch first demonstrated his postulates using Bacillus anthracis as the causative agent of anthrax in the 1870s, establishing the germ theory of disease. ---

Question 5

Which of the following statements about lipopolysaccharide is correct? A) It interacts with macrophages and monocytes yielding release of cytokines

Accepted Answer

** The toxic component is the O side chain** — LPS (endotoxin) from gram-negative bacteria binds to Toll-like receptor 4 on macrophages and monocytes, triggering cytokine release (TNF-α, IL-1, IL-6) that causes fever and septic shock. The toxic component is lipid A, not the O side chain. ---

Question 6

A 27-year-old man had rhinoplasty with nasal tampon placement. Eight hours later, he developed headache, muscle aches, abdominal cramps with diarrhea, and an erythematous rash resembling sunburn over his body including palms and soles. His blood pressure is 80/50 mm Hg. This illness was likely caused by: A) Lipopolysaccharide

Accepted Answer

** A toxin that is a superantigen** — This presentation is classic for Toxic Shock Syndrome (TSS). The superantigen toxin (TSST-1) causes massive T-cell activation and cytokine release, leading to the characteristic rash, hypotension, and multi-organ involvement. ---

Question 7

The organism most likely responsible for the patient's disease (Question 6) is: A) Escherichia coli

Accepted Answer

** Staphylococcus aureus — Staphylococcus aureus produces TSST-1, the superantigen toxin responsible for Toxic Shock Syndrome, particularly associated with tampon use and wound packing. ---

Question 8

Which of the following is most likely to be associated with bacterial biofilm formation? A) Airway colonization in a cystic fibrosis patient with mucoid Pseudomonas aeruginosa

Accepted Answer

** Urinary tract infection with *Escherichia coli*** — Mucoid P. aeruginosa strains produce alginate, which forms biofilms in the airways of cystic fibrosis patients, making infections extremely difficult to eradicate. ---

Question 9

Regarding bacterial type III secretion systems, which statement is correct? A) They are commonly found in gram-positive commensal bacteria

Accepted Answer

** They directly inject bacterial proteins into host cells across bacterial and host cell membranes** — Type III secretion systems are molecular syringes that directly inject virulence factors into host cells, bypassing extracellular defenses. They're found in gram-negative pathogens like Salmonella , Shigella , and Yersinia . ---

Question 10

Which of the following statements is correct? A) Lipopolysaccharide is part of the cell wall of Escherichia coli

Accepted Answer

** Cholera toxin is attached to the flagella of *Vibrio cholerae*** — LPS is a major component of the outer membrane of gram-negative bacteria like E. coli . Cholera toxin is not attached to flagella, lecithinase causes tissue necrosis (not diarrhea), and TSST-1 is produced by S. aureus , not S. epidermidis . ---

Question 11

A 15-year-old girl develops severe watery diarrhea that looks like "rice water." It is voluminous ( 1L in 90 minutes). She has no fever and seems otherwise normal except for fluid/electrolyte loss effects. The most likely cause is: A) Clostridium difficile enterotoxin

Accepted Answer

** A toxin with A and B subunits** — This classic presentation of "rice water" stools with massive fluid loss and no fever is pathognomonic for cholera. Cholera toxin has A and B subunits - the B subunit binds to GM1 ganglioside receptors, and the A subunit activates adenylyl cyclase. ---

Question 12

The most important treatment for the patient in Question 11 is: A) Give her ciprofloxacin

Accepted Answer

** Treat her with fluid and electrolyte replacement** — The primary treatment for cholera is aggressive fluid and electrolyte replacement (oral rehydration therapy or IV fluids). This addresses the main pathophysiology and can be life-saving. Antibiotics are secondary. ---

Question 13

A 23-year-old woman has recurrent urinary tract infections, including pyelonephritis. Blood typing shows P blood group antigen. The likely primary cause of her infections is: A) Escherichia coli that produce heat-stable toxin

Accepted Answer

** *Escherichia coli* with P-pili (fimbriae)** — P-pili allow E. coli to bind to P blood group antigens on uroepithelial cells, facilitating adherence and ascending infection to cause recurrent UTIs and pyelonephritis. The patient's P blood group makes her particularly susceptible. ---

Question 14

A 55-year-old man presents with weight loss, abdominal pain, diarrhea, and arthropathy. Small bowel biopsy shows periodic acid-Schiff-positive inclusions. Which test would confirm Whipple disease caused by Tropheryma whipplei ? A) Culture on agar media

Accepted Answer

** Polymerase chain reaction amplification and sequencing of appropriate DNA segment** — Tropheryma whipplei is extremely difficult to culture. PCR amplification and sequencing of 16S rRNA or other specific genes is the gold standard for diagnosis of Whipple disease. ---

Question 15

Which best describes the mechanism of action of diphtheria toxin? A) Forms pores in red blood cells causing hemolysis

Accepted Answer

** Inhibits elongation factor 2** — Diphtheria toxin ADP-ribosylates elongation factor 2 (eEF-2), blocking protein synthesis and leading to cell death. This mechanism is specific and highly toxic. ---

Question 16

A woman presents with vaginal discharge. Examination shows thin, homogeneous, white-gray discharge adhering to vaginal wall with pH 5.5. Gram stain shows epithelial cells covered with gram-variable rods. Which normal flora organism is greatly decreased in bacterial vaginosis? A) Corynebacterium species

Accepted Answer

** *Lactobacillus* species — Bacterial vaginosis is characterized by a decrease in lactobacilli (which normally maintain acidic pH) and overgrowth of anaerobes like Gardnerella vaginalis and Prevotella . The elevated pH ( 4.5) and clue cells confirm the diagnosis. ---

Question 17

Which microorganism is never considered normal flora and always considered pathogenic? A) Streptococcus pneumoniae

Accepted Answer

** *Mycobacterium tuberculosis*** — M. tuberculosis is always pathogenic and never part of normal flora. The other organisms can be found as normal flora in healthy individuals: S. pneumoniae in nasopharynx, E. coli in gut, S. aureus on skin/nose, N. meningitidis in nasopharynx. ---

Question 18

A 9-year-old girl has fever and severe right throat pain. Examination shows redness and swelling in right peritonsillar area. A peritonsillar abscess is diagnosed. The most likely organisms to be cultured are: A) Staphylococcus aureus

Accepted Answer

** Normal oral flora** — Peritonsillar abscesses are typically polymicrobial infections involving normal oral flora, including both aerobic and anaerobic bacteria such as streptococci, staphylococci, and anaerobes like Prevotella and Fusobacterium . ---

Question 19

Which of the following are vectors of typhoid pathogens? A) True flies

Accepted Answer

** *Haematogogus*** — House flies ( Musca domestica ) and other true flies can mechanically transmit Salmonella Typhi (typhoid fever) by contaminating food and water after contact with infected feces. ---

Question 20

Arboviruses are mainly transmitted by: A) Aedes species

Accepted Answer

** All of the above — Different arboviruses are transmitted by various arthropod vectors: Aedes (dengue, Zika, yellow fever), Culex (West Nile virus), and Culicoides (bluetongue virus). The term "arbovirus" means arthropod-borne virus. ---

Question 21

Anthropophilic vectors prefer: A) Animal hosts

Accepted Answer

** Human hosts** — "Anthropophilic" literally means "human-loving." These vectors preferentially feed on humans rather than animals. Examples include Aedes aegypti and human body lice. ---

Question 22

Exophagic insects prefer: A) Resting indoors

Accepted Answer

** C and D — "Exophagic" refers to outdoor feeding behavior, and such insects typically also rest outdoors (exophilic). This contrasts with endophagic insects that feed indoors. ---

Question 23

Presence of chrysops in the environment predisposes the community to which infections? A) River blindness

Accepted Answer

** Pink eye disease** — Chrysops species (deer flies) transmit Loa loa (African eye worm), which can cause temporary conjunctivitis and the characteristic "eye worm" migration across the conjunctiva. ---

Question 24

Which stage of filarial nematodes will you find in the chest muscles of their vectors? A) Larval stage 1

Accepted Answer

** Larval stage 3** — In the filarial life cycle, L3 larvae (infective stage) develop in the flight muscles of the insect vector and migrate to the proboscis, ready for transmission during the next blood meal. ---

Question 25

Pediculosis can be best treated with: A) Antibiotics

Accepted Answer

** Insecticides** — Pediculosis (lice infestation) is most effectively treated with topical insecticides like permethrin or malathion. Fine-toothed combs help remove nits but don't kill live lice effectively. ---

Question 26

Which of the following arthropods is most likely to be monophagous? A) Louse

Accepted Answer

** Flea** — Lice are monophagous (feed on one host species). Human lice are highly host-specific. Other options are more polyphagous, feeding on multiple host species. ---

Question 27

Aedes albopictus is mainly found in: A) Asia

Accepted Answer

** Africa** — Aedes albopictus (Asian tiger mosquito) is native to Southeast Asia, though it has now spread globally. It's an important vector for dengue and chikungunya viruses. ---

Question 28

Mosquito life cycle has how many larval instars? A) One

Accepted Answer

** Four** — Mosquitoes have four larval instars (L1-L4) before pupation. Each instar involves molting and growth before the final molt to the pupal stage. ---

Question 29

Which one of the following is a repellant? A) DDT

Accepted Answer

** DEET** — DEET (N,N-diethyl-meta-toluamide) is the most effective insect repellent. The others are insecticides: DDT (organochlorine), permethrin (pyrethroid), and bendiocarb (carbamate). ---

Question 30

Which one has the GABA site as its target? A) DDT

Accepted Answer

** Permethrin** — Permethrin (pyrethroid) targets GABA-gated chloride channels in insect neurons. Bendiocarb targets acetylcholinesterase, malathion targets the same, and DDT targets sodium channels. ---

Question 31

Presence of anthills could predispose the population to: A) Sleeping sickness

Accepted Answer

** Leishmaniasis** — Sandflies, vectors of leishmaniasis, often breed in organic-rich soil around anthills and burrows. The proximity increases sandfly populations and disease transmission risk. ---

Question 32

People living in forested areas like Arabuko Sokoke are at risk of: A) Sleeping sickness

Accepted Answer

** Leishmaniasis** — Forested areas are habitats for tsetse flies ( Glossina species), which transmit African trypanosomiasis (sleeping sickness). Arabuko Sokoke forest in Kenya is a known tsetse habitat. ---

Question 33

Which vectors transmit Trypanosome parasites in Latin America? A) Glossina species

Accepted Answer

** Triatomine bugs** — American trypanosomiasis (Chagas disease) is transmitted by triatomine bugs (kissing bugs). Glossina transmits African trypanosomiasis, not American. ---

Question 34

Pruritus can be best diagnosed for: A) Sarcoptes scabiei

Accepted Answer

** *Leptothrombidium* species** — Sarcoptes scabiei (scabies mite) causes intense pruritus (itching), especially at night. The mites burrow in the skin, causing characteristic itchy lesions and secondary bacterial infections from scratching. ---

Question 35

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

Accepted Answer

** *Pediculus humanus*** — Pediculus humanus (body louse) lives in clothing and lays eggs (nits) in clothing seams. Head lice ( P. capitis ) lay nits on hair shafts, and crab lice ( Pthirus pubis ) lay nits on pubic hair. --- # SECTION B: SHORT ANSWER QUESTIONS (25 MARKS)

Question 36

(20 marks) Investigation of urinary tract infections is common in diagnostic microbiology laboratory.

Accepted Answer

Describe the routes of urinary tract infection and the organism commonly associated with each mode of transmission (8 marks): 1. Ascending Route (Most Common) (3 marks): - Mechanism: Bacteria ascend from urethra to bladder, ureters, and kidneys - Common organisms: *Escherichia coli* (80-85% of uncomplicated UTIs) - *Klebsiella pneumoniae* - *Enterococcus faecalis* - *Proteus mirabilis* 2. Hematogenous Route (2 marks): - Mechanism: Bacteria reach kidneys via bloodstream - Common organisms: *Staphylococcus aureus* - *Candida* species - *Mycobacterium tuberculosis* 3. Lymphatic Route (1 mark): - Mechanism: Spread via lymphatic system from adjacent organs - Less common route 4. Direct Extension (2 marks): - Mechanism: Direct spread from adjacent infected organs - Common organisms: *Enterobacter* species from bowel - Mixed anaerobic bacteria from pelvic infections

Medical Bacteriology and Entomology | MCQ Quiz | OmpathStudy Kenya

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