USMLE Step 1 Quiz: Bacterial Structure Classification And Virulence

A 62-year-old patient with AIDS (CD4 count of 40 cells/μL) develops a disseminated infection with Mycobacterium avium complex (MAC). He presents with fevers, night sweats, weight loss, and diarrhea. The organism is found in high numbers within macrophages throughout his reticuloendothelial system.

The ability of this organism to survive and replicate within phagocytic cells is primarily due to which feature of its structure?

A 35-year-old immigrant from Southeast Asia presents with a chronic cough, night sweats, weight loss, and hemoptysis. A sputum sample is collected and subjected to a special staining procedure, which reveals red-staining rod-shaped organisms against a blue background. The bacterium's ability to resist decolorization by acid-alcohol is a key diagnostic feature.

This staining property is primarily attributed to the high concentration of which substance in the bacterium's cell envelope?

A 19-year-old man presents to a clinic with a purulent urethral discharge and dysuria. A Gram stain of the discharge shows intracellular Gram-negative diplococci. The causative organism, Neisseria gonorrhoeae, has an outer membrane that contains channels allowing for the passage of hydrophilic molecules.

Which of the following structures forms these channels in the outer membrane?

A 68-year-old man with a history of an indwelling urinary catheter is admitted to the intensive care unit with a fever of 39.5°C (103.1°F), a heart rate of 120/min, and blood pressure of 85/50 mm Hg. Blood cultures grow a Gram-negative rod. The patient develops disseminated intravascular coagulation (DIC). The physician explains that a component of the bacterial outer membrane is responsible for these systemic effects.

Which of the following components is the principal mediator of septic shock in this patient?

An 80-year-old asplenic man is brought to the emergency department with fever, headache, and altered mental status. His condition rapidly deteriorates, and he develops septic shock. Blood cultures grow Streptococcus pneumoniae. Asplenic individuals are particularly susceptible to severe infections with this organism.

The increased virulence of this pathogen in asplenic patients is primarily due to the loss of splenic clearance of organisms possessing which of the following structures?

A patient in a long-term care facility develops a complicated urinary tract infection with a strain of Pseudomonas aeruginosa that is resistant to multiple classes of antibiotics, including carbapenems. Genetic analysis of the isolate reveals that the resistance genes are located on a small, circular, extrachromosomal piece of DNA.

Which of the following bacterial structures is the most likely location for these multi-drug resistance genes?

A 24-year-old woman presents to the emergency department with fever, hypotension, and a diffuse erythematous rash resembling sunburn. Her medical history is significant for menstruation, and she reports using super-absorbent tampons. Blood cultures are drawn, and she is started on intravenous fluids and broad-spectrum antibiotics. The pathogenesis of her condition is attributed to a bacterial exotoxin that acts as a superantigen. This toxin nonspecifically links T-cell receptors to MHC class II molecules on antigen-presenting cells.

The massive cytokine release seen in this patient is a direct result of activating which component of the bacterial cell or its products?

A 20-year-old college student living in a dormitory presents with a persistent, non-productive cough, low-grade fever, and malaise for the past three weeks. A chest X-ray reveals a patchy interstitial infiltrate. Sputum Gram stain shows numerous neutrophils but no organisms. The patient is prescribed a beta-lactam antibiotic, but his symptoms do not improve. The suspected organism is known for its lack of a rigid cell wall.

The resistance of the causative organism to the prescribed antibiotic is due to the absence of which of the following structures?

A 55-year-old farmer sustains a deep, contaminated wound from a piece of farm equipment. The wound is not properly cleaned. Several days later, he develops severe pain, swelling, and a foul-smelling discharge from the wound. On examination, crepitus is felt in the subcutaneous tissue. The causative organism, Clostridium perfringens, is known for its ability to survive in harsh environmental conditions, such as soil, for prolonged periods.

Which of the following bacterial structures is most responsible for the organism's long-term survival in the environment?

A 60-year-old man with a history of alcohol use disorder is brought to the hospital with the abrupt onset of high fever, chills, and productive cough with thick, blood-tinged sputum described as 'currant jelly'. A chest X-ray shows lobar consolidation with a bulging fissure in the right upper lobe. The causative organism, Klebsiella pneumoniae, is known for its prominent virulence factor that gives its colonies a mucoid appearance.

Which of the following structures is the primary virulence factor responsible for the characteristic sputum and the organism's resistance to phagocytosis?

A 22-year-old sexually active woman presents with a 3-day history of dysuria, urinary frequency, and urgency. A urine culture grows more than 100,000 colony-forming units/mL of a Gram-negative rod identified as E. coli. The pathogenesis of this infection begins with the bacterium's ability to colonize the urinary tract.

Which bacterial structure is most critical for the initial attachment of this organism to uroepithelial cells?

A 45-year-old man presents with a 2-month history of gnawing epigastric pain that improves with meals. An endoscopy reveals a duodenal ulcer. A biopsy of the gastric antrum is taken, and a urease test is positive. The causative organism, Helicobacter pylori, is able to survive and move within the highly acidic stomach environment.

Which of the following structures is essential for the bacterium's motility through the viscous gastric mucus to reach the underlying epithelium?

A 40-year-old intravenous drug user presents with high fever, chills, and a new heart murmur. An echocardiogram reveals vegetations on the tricuspid valve, consistent with infective endocarditis. Blood cultures grow Gram-positive cocci in clusters. A key virulence factor for the adherence of this organism to heart valves involves an anionic polymer in its cell wall.

Which of the following cell wall components of Staphylococcus aureus is most directly involved in mediating attachment to host fibronectin on damaged heart valves?

A 19-year-old college student presents with fever, severe headache, nuchal rigidity, and a petechial rash. A lumbar puncture is performed, and cerebrospinal fluid analysis is consistent with bacterial meningitis. The causative organism is a Gram-negative diplococcus that colonizes the nasopharynx. A key virulence factor allows it to evade host defenses on mucosal surfaces.

Which of the following virulence factors produced by Neisseria meningitidis is specifically designed to cleave secretory IgA?

A medical student is studying bacterial cell walls in a microbiology lab. He learns that the differential staining in the Gram stain procedure is based on the structural differences between Gram-positive and Gram-negative cell envelopes.

Which of the following best explains why Gram-positive bacteria retain the crystal violet stain while Gram-negative bacteria do not?

A 45-year-old bird breeder presents with an abrupt onset of fever, chills, severe headache, and a non-productive cough. A chest X-ray shows patchy infiltrates. His occupational history is highly suggestive of psittacosis. The causative agent, Chlamydophila psittaci, has a unique developmental cycle involving an infectious elementary body and a replicative reticulate body.

The cell wall of this organism is unusual because it lacks which of the following components found in most other bacteria?

A patient is treated for a severe bacterial infection with azithromycin, an antibiotic that binds to the 50S ribosomal subunit and inhibits translocation, thereby blocking protein synthesis. This drug is effective against bacteria but does not harm human cells.

This selective toxicity is based on a fundamental difference in which of the following structures between prokaryotes and eukaryotes?

A 1-week-old infant is brought to the pediatrician with diffuse erythema and large, flaccid bullae. Gentle pressure on the skin causes the epidermis to separate and slough off (Nikolsky sign). The infant is diagnosed with staphylococcal scalded skin syndrome (SSSS). The pathogenesis involves a bacterial toxin that targets a specific protein in the epidermis.

The widespread blistering in this condition is caused by an exotoxin that specifically cleaves which of the following adhesion molecules?

A 34-year-old hiker returns from a trip in North Carolina and develops fever, myalgia, and a severe headache. A few days later, a maculopapular rash appears on his wrists and ankles and spreads centripetally to his trunk, palms, and soles. A diagnosis of Rocky Mountain spotted fever is made. The causative agent, Rickettsia rickettsii, has a specific requirement for growth.

Based on its classification, which of the following is a defining characteristic of this organism's life cycle?

A 7-year-old boy presents with fever and a sore throat. On examination, his pharynx is erythematous with a grayish-white exudate, and he has tender anterior cervical lymphadenopathy. A rapid antigen test is positive for Streptococcus pyogenes. The physician is concerned about the potential for systemic complications related to a specific exotoxin.

Which virulence factor produced by certain strains of this organism is a superantigen responsible for the rash of scarlet fever and the systemic toxicity of streptococcal toxic shock syndrome?