Medical Bacteriology and Entomology – 35 MCQs | Kenya MBChB

35 Year 2: Microbiology exam questions on Medical Bacteriology and Entomology for medical students. Includes MCQs, answers, explanations and written questions.

This MCQ set contains 35 questions on Medical Bacteriology and Entomology in the Year 2: Microbiology unit. Each question includes the correct answer and a detailed explanation for active recall and exam preparation.

Q1: 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 a

  1. A. Ingesting the microorganisms in her food
  2. B. Holding onto contaminated hand rails when she takes public transportation
  3. C. Handling potting soil
  4. D. Breathing aerosolized droplets containing the microorganism

Correct answer: D – 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. ---

Q2: 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

  1. A. Vibrio cholerae type O139
  2. B. Shigella dysenteriae type 1
  3. C. Campylobacter jejuni
  4. D. Entamoeba histolytica

Correct answer: A – 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. ---

Q3: 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

  1. A. Production of extracellular polysaccharide inhibits growth of S. epidermidis
  2. B. The S. epidermidis in the biofilm are more susceptible to antimicrobial therapy
  3. C. The quorum-sensing ability results in increased susceptibility to antimicrobial therapy
  4. D. The complex molecular interactions within the biofilm make it difficult to provide effective antimicrobial therapy, and the catheter will likely need

Correct answer: D – The complex molecular interactions within the biofilm make it difficult to provide effective antimicrobial therapy, and the catheter will likely need

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. ---

Q4: The first microorganism to satisfy Koch's postulates (in the late 19th century) was: A) Treponema pallidum

  1. A. Stenotrophomonas maltophilia
  2. B. Mycobacterium leprae
  3. C. Bacillus anthracis
  4. D. Neisseria gonorrhoeae

Correct answer: C – 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. ---

Q5: Which of the following statements about lipopolysaccharide is correct? A) It interacts with macrophages and monocytes yielding release of cytokines

  1. A. The toxic component is the O side chain
  2. B. It forms holes in red blood cell membranes yielding hemolysis
  3. C. It causes hypothermia
  4. D. It causes paralysis

Correct answer: A – 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. ---

Q6: 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 cause

  1. A. Peptidoglycan
  2. B. A toxin that is a superantigen
  3. C. A toxin that has A and B subunits
  4. D. Lecithinase (alpha toxin)

Correct answer: B – 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. ---

Q7: The organism most likely responsible for the patient's disease (Question 6) is: A) Escherichia coli

  1. A. Corynebacterium diphtheriae
  2. B. Clostridium perfringens
  3. C. Neisseria meningitidis
  4. D. Staphylococcus aureus

Correct answer: D – Staphylococcus aureus

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

Q8: 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

  1. A. Urinary tract infection with Escherichia coli
  2. B. Meningitis with Neisseria meningitidis
  3. C. Tetanus
  4. D. Impetigo caused by Staphylococcus aureus

Correct answer: A – 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. ---

Q9: Regarding bacterial type III secretion systems, which statement is correct? A) They are commonly found in gram-positive commensal bacteria

  1. A. They play an important role in the pathogenesis of diseases caused by Clostridium species
  2. B. They cause release of effectors into the extracellular environment
  3. C. They directly inject bacterial proteins into host cells across bacterial and host cell membranes
  4. D. Mutations that prevent functioning enhance pathogenesis

Correct answer: C – 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 . ---

Q10: 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

  1. A. A toxin with A and B subunits
  2. B. Shigella dysenteriae type 1 that produces Shiga toxin
  3. C. Enterotoxigenic Escherichia coli that produces heat-labile and heat-stable toxins
  4. D. Staphylococcal enterotoxin F

Correct answer: A – 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. ---

Q11: The most important treatment for the patient in Question 11 is: A) Give her ciprofloxacin

  1. A. Give her a toxoid vaccine
  2. B. Give her the appropriate antitoxin
  3. C. Treat her with fluid and electrolyte replacement
  4. D. Culture her stool to make the correct diagnosis then treat specifically

Correct answer: C – 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. ---

Q12: 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

  1. A. Escherichia coli with K1 (capsular type 1) antigen
  2. B. Escherichia coli O139
  3. C. Escherichia coli with P-pili (fimbriae)
  4. D. Escherichia coli O157:H7

Correct answer: C – 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. ---

Q13: 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

  1. A. Polymerase chain reaction amplification and sequencing of appropriate DNA segment
  2. B. Cocultivation with Escherichia coli
  3. C. In situ hybridization
  4. D. Direct fluorescent antibody test

Correct answer: A – 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. ---

Q14: Which best describes the mechanism of action of diphtheria toxin? A) Forms pores in red blood cells causing hemolysis

  1. A. Degrades lecithin in eukaryotic cell membranes
  2. B. Causes release of tumor necrosis factor
  3. C. Inhibits elongation factor 2
  4. D. Causes increased adenylate cyclase activity

Correct answer: C – 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. ---

Q15: 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 spec

  1. A. Staphylococcus epidermidis
  2. B. Prevotella species
  3. C. Candida albicans
  4. D. Lactobacillus species

Correct answer: D – 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. ---

Q16: Which microorganism is never considered normal flora and always considered pathogenic? A) Streptococcus pneumoniae

  1. A. Escherichia coli
  2. B. Mycobacterium tuberculosis
  3. C. Staphylococcus aureus
  4. D. Neisseria meningitidis

Correct answer: B – 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. ---

Q17: 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

  1. A. Streptococcus pneumoniae
  2. B. Corynebacterium species and Prevotella melaninogenica
  3. C. Normal oral flora
  4. D. Viridans streptococci and Candida albicans

Correct answer: C – 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 . ---

Q18: Which of the following are vectors of typhoid pathogens? A) True flies

  1. A. Haematogogus
  2. B. Sabathes
  3. C. Fannia
  4. D. Stomoxys

Correct answer: A – 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. ---

Q19: Arboviruses are mainly transmitted by: A) Aedes species

  1. A. Culex species
  2. B. Cimex species
  3. C. Culicoides species
  4. D. All of the above

Correct answer: D – 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. ---

Q20: Anthropophilic vectors prefer: A) Animal hosts

  1. A. Human hosts
  2. B. Bird hosts
  3. C. Amphibian hosts
  4. D. Any of the above

Correct answer: A – Human hosts

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

Q21: Exophagic insects prefer: A) Resting indoors

  1. A. Feeding indoors
  2. B. Resting outdoors
  3. C. Feeding outdoors
  4. D. C and D

Correct answer: D – 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. ---

Q22: Presence of chrysops in the environment predisposes the community to which infections? A) River blindness

  1. A. Pink eye disease
  2. B. Trachoma
  3. C. Lymphedema
  4. D. Conjunctivitis

Correct answer: A – 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. ---

Q23: Which stage of filarial nematodes will you find in the chest muscles of their vectors? A) Larval stage 1

  1. A. Larval stage 2
  2. B. Larval stage 3
  3. C. Larval stage 4
  4. D. Microfilariae

Correct answer: B – 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. ---

Q24: Pediculosis can be best treated with: A) Antibiotics

  1. A. A fine-toothed comb
  2. B. Insecticides
  3. C. Vaseline jelly
  4. D. All of the above

Correct answer: B – 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. ---

Q25: Which of the following arthropods is most likely to be monophagous? A) Louse

  1. A. Flea
  2. B. Culex
  3. C. Tick
  4. D. Anopheles

Correct answer: A – 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. ---

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