Medical Bacteriology MCQs: Review Strep, TB, Anthrax & More

Question 1

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

Accepted Answer

Rheumatic fever — Streptococcal pharyngitis (Group A strep) can trigger an autoimmune cross-reaction against heart tissue weeks later, causing rheumatic fever. The other options belong to different organisms — diphtheria (pseudomembrane), TB (tubercles), and N. meningitidis (meningococcemia/Waterhouse-Friderichsen). --- ---

Question 2

The presence of a pseudomembrane is a result of

Accepted Answer

Strep throat — Corynebacterium diphtheriae toxin kills epithelial cells, and the resulting dead cells, fibrin, and inflammatory cells form the characteristic grayish pseudomembrane over the tonsils/pharynx. --- ---

Question 3

A paroxysm is a and is characteristic of .

Accepted Answer

fever; meningitis — The paroxysmal stage of whooping cough ( Bordetella pertussis ) is defined by bursts of rapid, forceful coughing, often ending in a characteristic "whoop" inspiration. --- ---

Question 4

The Mantoux test is used to identify

Accepted Answer

TB exposure — The Mantoux (tuberculin) skin test detects prior exposure/sensitization to M. tuberculosis antigens via a delayed-type hypersensitivity reaction (induration), not active disease. --- ---

Question 5

This bacterial species is the most common cause of "typical" pneumonia.

Accepted Answer

H. influenzae — Streptococcus pneumoniae is the leading cause of classic (lobar/typical) community-acquired pneumonia, distinguished clinically from "atypical" pneumonias caused by Mycoplasma or Legionella . --- ---

Question 6

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

Accepted Answer

antibiotic — Botulism is caused by a preformed toxin, not an active bacterial infection at the point of ingestion, so treatment is passive immunization with antitoxin to neutralize circulating toxin. --- ---

Question 7

The organism Clostridium perfringens multiplies in foods only under conditions.

Accepted Answer

aerobic — ---

Question 8

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

Accepted Answer

anthracis — Bacillus cereus causes two distinct food poisoning syndromes — a diarrheal type (heat-labile toxin, meats/sauces) and an emetic type (heat-stable toxin, classically reheated fried rice). --- ---

Question 9

Woolsorter disease applies to the form of .

Accepted Answer

inhalation; tularemia — "Woolsorter's disease" is the historical name for inhalational anthrax, contracted by workers exposed to spore-contaminated animal wool/hair. --- ---

Question 10

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

Accepted Answer

It inhibits muscle contraction — Tetanospasmin blocks release of the inhibitory neurotransmitters glycine and GABA at spinal synapses. Without inhibitory signals, muscles cannot relax, causing sustained rigid/spastic contraction (lockjaw, opisthotonus). --- ---

Question 11

Salpingitis is associated with and can lead to .

Accepted Answer

syphilis; gumma formation — Neisseria gonorrhoeae ascending infection can cause salpingitis (fallopian tube inflammation), scarring the tubes and causing infertility/sterility. --- ---

Question 12

Ophthalmia is associated with what two STDs?

Accepted Answer

Syphilis and chlamydia — Both N. gonorrhoeae and Chlamydia trachomatis can cause ophthalmia neonatorum, transmitted to a newborn during vaginal delivery — this is why both silver nitrate/erythromycin prophylaxis is given at birth. --- ---

Question 13

Toxin-generated S. aureus diseases resulting from contact include

Accepted Answer

purpuric fever — Both scalded skin syndrome (exfoliative toxin) and toxic shock syndrome (TSST-1) are toxin-mediated S. aureus diseases; pseudomembranous colitis is caused by ---

Question 14

A 22-year-old woman who works in a plant nursery presents with fever/cough for 2 months, weight loss, bilateral upper lobe infiltrates with cavities, acid-fast bacilli on sputum. Likely means of acquiring infection

Accepted Answer

Sexual activity — Acid-fast bacilli with cavitary upper-lobe disease indicates pulmonary TB, acquired via inhalation of airborne droplet nuclei — the soil-exposure detail is a distractor, not the transmission route for T B. --- ---

Question 15

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

Accepted Answer

Treponema pallidum — Robert Koch used ---

Question 16

Which of the following statements about lipopolysaccharide (LPS) is correct?

Accepted Answer

It interacts with macrophages and monocytes yielding release of cytokines — LPS (endotoxin) triggers cytokine release via macrophage/monocyte receptors (TLR4). The toxic component is actually lipid A, not the O side chain (which is the antigenic/serotyping portion); LPS doesn't lyse RBCs or directly cause hypothermia/paralysis. --- ---

Question 17

Which of the following statements is correct?

Accepted Answer

Lipopolysaccharide is part of the cell wall of Escherichia coli — LPS is a genuine structural component of the gram-negative outer membrane. Cholera toxin is a separately secreted AB toxin, not flagella-attached; lecithinase causes tissue necrosis, not diarrhea (a separate enterotoxin does that); TSST-1 comes from S. aureus , not S. epidermidis . --- ---

Question 18

Which of the following best describes the mechanism of action of diphtheria toxin?

Accepted Answer

Forms pores in red blood cells causing hemolysis — The toxin's A subunit ADP-ribosylates EF-2, halting host cell protein synthesis and causing cell death — this underlies the pseudomembrane formation and systemic toxicity (e.g., cardiotoxicity). --- ---

Question 19

Certain microorganisms are never considered to be members of the normal flora — always pathogens. Which fits this category?

Accepted Answer

Streptococcus pneumoniae — Unlike the others, which can exist as normal/carrier flora in some individuals, M. tuberculosis is always considered a true pathogen when found. --- ---

Question 20

A 27-year-old man had rhinoplasty with a nasal tampon. He develops headache, muscle aches, diarrhea, an erythematous "sunburn" rash including palms/soles, hypotension (80/50), elevated liver enzymes, and renal failure. This illness is likely caused by

Accepted Answer

Lipopolysaccharide — This is classic Toxic Shock Syndrome from a retained foreign body (nasal tampon), caused by TSST-1, a superantigen that causes massive, non-specific T-cell activation and cytokine storm. --- ---

Question 21

The organism most likely responsible for the patient's disease in Question 21 is

Accepted Answer

Escherichia coli — TSST-1, the toxin implicated in Question 21, is produced by S. aureus . --- ---

Question 22

Which of the following is most likely to be associated with the formation of a bacterial biofilm?

Accepted Answer

Airway colonization in a cystic fibrosis patient with a mucoid (alginate-producing) strain of Pseudomonas aeruginosa — Mucoid, alginate-producing P. aeruginosa in CF airways is the textbook example of clinically significant biofilm formation, protecting the organism from both antibiotics and host immune clearance. --- ---

Question 23

Regarding bacterial type III secretion systems, which statement is correct?

Accepted Answer

Directly inject bacterial proteins into host cells across bacterial and host cell membranes, promoting pathogenesis — Type III secretion systems form a molecular "syringe" that injects effector proteins directly into host cells, a mechanism used by gram-negative pathogens like Salmonella , Shigella , and Yersinia — not the gram-positive Clostridium toxin diseases in option B. --- ---

Question 24

A 15-year-old girl develops severe watery "rice water" diarrhea, 1L in the last 90 minutes, no fever. Most likely cause of her illness

Accepted Answer

Clostridium difficile enterotoxin — This describes classic cholera ( Vibrio cholerae ); cholera toxin is an AB-subunit toxin that massively increases intracellular cAMP, driving profuse watery secretory diarrhea. --- ---

Question 25

The most important thing that can be done to treat the patient in Question 25 is

Accepted Answer

Give her ciprofloxacin — In cholera, death results from dehydration, not the bacteria/toxin directly — rehydration is the immediately life-saving priority, taking precedence over antibiotics or diagnostic confirmation. --- ---

Question 26

A 23-year-old woman with recurrent UTIs, at least one episode of pyelonephritis. Blood typing shows the P blood group antigen. Likely primary cause of her infections

Accepted Answer

Escherichia coli producing heat-stable toxin — P-pili (fimbriae) bind to a receptor related to the P blood group antigen on uroepithelial cells, mediating attachment and ascending infection to the kidneys (pyelonephritis). --- ---

Question 27

A 55-year-old man with weight loss, abdominal pain, diarrhea, and arthropathy; small bowel biopsy shows PAS-positive inclusions in the bowel wall. Which test could confirm Whipple disease, caused by Tropheryma whipplei?

Accepted Answer

Polymerase chain reaction (PCR) amplification and sequencing of an appropriate DNA segment — T. whipplei is very difficult to culture on standard media, so molecular methods (PCR/sequencing) are the standard confirmatory diagnostic approach. --- ---

Question 28

- Vaginal discharge, thin homogeneous white-gray discharge adhering to vaginal wall, pH 5.5, epithelial cells covered with gram-variable rods (bacterial vaginosis). Which normal genital flora organism is greatly decreased?

Accepted Answer

Corynebacterium species — In bacterial vaginosis, protective Lactobacillus (which maintains acidic vaginal pH) is depleted and replaced by an overgrowth of anaerobes like Gardnerella and Prevotella , raising vaginal pH above 4.5. --- ---

Question 29

A 9-year-old girl develops fever and severe right-sided throat pain; redness/swelling in the right peritonsillar area — peritonsillar abscess diagnosed. Most likely organisms cultured

Accepted Answer

Staphylococcus aureus — Peritonsillar abscesses are typically polymicrobial, often involving anaerobes like Prevotella melaninogenica alongside other oropharyngeal flora. --- ---

Question 30

Which of the following are vectors of typhoid pathogens?

Accepted Answer

True flies — True flies ( Musca domestica ) mechanically transmit Salmonella typhi by contaminating food/water; Haematogogus and Sabethes are actually yellow fever mosquito vectors, not typhoid vectors. --- ---

Question 31

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

Nose — The anterior nares are the classic carriage site (reservoir) for S. aureus in both patients and healthcare workers, and a common outbreak source in nurseries. --- ---

Question 32

Each of the following organisms is an important cause of urinary tract infections except

Accepted Answer

Klebsiella pneumoniae — Bacteroides fragilis is an anaerobe associated with intra-abdominal infections/abscesses, not a typical UTI pathogen; the other three are classic uropathogens. --- ---

Question 33

A 30-year-old woman has non-bloody diarrhea for the past 14 hours. Which organism is LEAST likely to cause this illness?

Accepted Answer

Streptococcus pyogenes — S. pyogenes is not a recognized cause of infectious diarrhea; it causes pharyngitis, skin/soft tissue infections, and their immune sequelae, not gastroenteritis. --- ---

Question 34

Scalded skin syndrome is caused by exotoxin (exfoliatins) produced by

Accepted Answer

Streptococcus pyrogen — Exfoliative toxins produced by certain S. aureus strains cleave desmoglein in the epidermis, causing widespread skin blistering/peeling (scalded skin appearance), most common in infants. --- ---

Question 35

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

Accepted Answer

Campylobacter jejuni — Shigella species have an unusually low infective dose (as few as 10–100 organisms), explaining their high person-to-person transmissibility compared to Salmonella or Vibrio , which need much larger inocula. --- ---

Question 36

Which of the following disease is best diagnosed by serologic means?

Accepted Answer

Pulmonary tuberculosis — Q fever ( Coxiella burnetii ) is difficult to culture safely and is typically diagnosed serologically; TB, gonorrhea, and actinomycosis are diagnosed by staining/culture-based methods instead. --- ---

Question 37

The survival of Mycobacterium after ingestion by macrophages is attributed to

Accepted Answer

Bacterial inhibition of phagosome formation and interference with endosomal acidification — M. tuberculosis survives inside macrophages primarily by blocking phagosome-lysosome fusion and preventing acidification of the phagosome, avoiding destruction rather than escaping the cell entirely (which is more characteristic of Listeria ). --- ---

Question 38

The molecular basis for the effect of cholera toxin on duodenal mucosa cells is

Accepted Answer

Inactivation of G1 protein — Cholera toxin ADP-ribosylates a G-protein, locking adenylate cyclase in an active state, which massively raises intracellular cAMP and drives chloride/water secretion into the gut lumen (option E describes the mechanism, but B describes the net physiological effect that's typically the marked answer in this exam's key). --- ---

Question 39

The role of bacterial capsule as a virulence factor is usually related to its ability to interfere with

Accepted Answer

Antibacterial penetration of bacterial cells — Capsules are primarily antiphagocytic — they prevent opsonized bacteria from being effectively recognized and engulfed by phagocytes. --- ---

Question 40

Which of the following is NOT the most important cause of acute bacterial meningitis?

Accepted Answer

Streptococcus pneumoniae — S. pyogenes is not a major cause of bacterial meningitis; the other three (pneumococcus, meningococcus, H. influenzae ) are classic major causes depending on age group. --- ---

Question 41

A 21-year-old married woman with a UTI, febrile, painful urination, flank pain. Urine culture: lactose-fermenting, indole-positive, gram-negative bacillus. Effectiveness of the causative organism in UTI is associated specifically with

Accepted Answer

Exotoxins — This organism is ---

Question 42

The coagulase test is used to differentiate

Accepted Answer

Staphylococcus aureus from Staphylococcus epidermidis — S. aureus is coagulase-positive; S. epidermidis is coagulase-negative — this is the standard test distinguishing the two clinically important staphylococci. --- ---

Question 43

Snails are both first and second intermediate hosts of?

Accepted Answer

Echinostoma ilocaanum — Unlike most flukes where snails are only the first intermediate host, Echinostoma ilocaanum uses snails for both the first and second intermediate host stages — a specifically tested exception. --- ---

Question 44

How can some millipedes cause medical problems?

Accepted Answer

Stinging — Millipedes release chemical defensive secretions that cause skin irritation on contact — they don't sting, transmit viruses, or lay eggs in skin. --- ---

Question 45

Rift Valley fever is mainly transmitted by?

Accepted Answer

Culex species — Aedes mosquitoes are the primary vectors of Rift Valley fever virus, along with other arboviruses. --- ---

Question 46

Parthenogenesis is a type of?

Accepted Answer

Asexual reproduction in which offspring develops from unfertilized eggs — Parthenogenesis is asexual reproduction where offspring develop directly from an unfertilized egg — no mating required. --- ---

Question 47

Elek's test is used to test

Accepted Answer

Lactose fermentation — Elek's test confirms whether a Corynebacterium diphtheriae isolate produces toxin, by observing a precipitation line where secreted toxin meets antitoxin-impregnated filter paper on the agar. --- ---

Question 48

Type of exotoxin most commonly associated with botulism in man is/are

Accepted Answer

Type A — Types A, B, and E are the botulinum toxin types most commonly implicated in human disease (types C and D mainly affect animals). --- ---

Question 49

Which of the following pathogen is typically acquired from another individual and can trigger rheumatic fever?

Accepted Answer

Mycoplasma pneumoniae — Rheumatic fever is a delayed autoimmune complication specifically of S. pyogenes (Group A strep) pharyngitis. --- ---

Question 50

The specific action of hemolysis is

Accepted Answer

Cause leukocytosis — Hemolysis, by definition, refers to lysis/destruction of red blood cells, as produced by hemolysins like streptolysin O. --- ---

Medical Bacteriology MCQs: Review Strep, TB, Anthrax & More

Questions, Answers & Explanations