Skin Microbiome — MCQs
Which of the following disorders would be more likely associated with Staphylococcus saprophyticus rather than Staphylococcus aureus?
It is true regarding normal microbial flora present on skin and mucous membrane that
An 18-year-old man has facial and upper back lesions that have waxed and waned for the past 6 years. On physical examination, there are 0.3- to 0.9-cm comedones, erythematous papules, nodules, and pustules most numerous on the lower face and posterior upper trunk. Other family members have been affected by this condition at a similar age. The lesions worsen during a 5-day cruise to the Adriatic. Which of the following organisms is most likely to play a key role in the pathogenesis of these lesions?
An organism produces cutaneous disease (malignant pustule or eschar) at the site of inoculation in handlers of animal skins. Most likely organism is:
A farmer presents with a subcutaneous wound on his foot with discharge. Microscopy of a white granule from the wound shows Gram-positive filamentous rods. What is the most likely organism?
"Isomorphic response" can be a feature of the following except
Which of the following is a false statement about the respective fungal infections?
All of the following statements about Staphylococcus aureus are true, except:
Botryomycosis is a ___ disease
Subdural empyema is most commonly caused by:
Skin Microbiome Indian Medical PG Practice Questions and MCQs
Question 1: Which of the following disorders would be more likely associated with Staphylococcus saprophyticus rather than Staphylococcus aureus?
- A. Burns
- B. Tension pneumothorax
- C. Osteomyelitis
- D. Acute cystitis (Correct Answer)
Explanation: ***Acute cystitis*** - **Staphylococcus saprophyticus** is a common cause of **urinary tract infections (UTIs)**, particularly acute cystitis, in young sexually active women. - This bacterium has a high affinity for **uroepithelial cells**, facilitating its colonization and subsequent infection of the bladder. *Tension pneumothorax* - A **tension pneumothorax** is a medical emergency characterized by air accumulation in the pleural space, leading to lung collapse and mediastinal shift. - It is typically caused by trauma or iatrogenic factors, not directly by bacterial infection from either *Staphylococcus saprophyticus* or *Staphylococcus aureus*. *Burns* - Burn wounds are highly susceptible to bacterial colonization and infection, with **Staphylococcus aureus** being a primary pathogen in this context. - *Staphylococcus saprophyticus* is rarely associated with burn wound infections. *Osteomyelitis* - **Osteomyelitis**, an infection of the bone, is most frequently caused by **Staphylococcus aureus** via hematogenous spread or direct inoculation. - *Staphylococcus saprophyticus* is not a common pathogen in osteomyelitis.
Question 2: It is true regarding normal microbial flora present on skin and mucous membrane that
- A. It is difficult to permanently eradicate by antimicrobial agents (Correct Answer)
- B. It is absent in the stomach due to acidic pH
- C. It establishes in the body only after the neonatal period
- D. The flora in the small bronchi is similar to that of the trachea
Explanation: ***It is difficult to permanently eradicate by antimicrobial agents*** - The **normal flora** colonizes various body sites, often forming **biofilms** or residing in protected niches, making complete eradication challenging even with potent antimicrobials. - While antimicrobials can reduce microbial populations, the remaining organisms, or recolonization from external sources, can lead to the re-establishment of the flora. *It is absent in the stomach due to acidic pH* - The stomach is not entirely sterile; **acid-tolerant bacteria**, such as *Helicobacter pylori*, can colonize the gastric mucosa. - While the **acidic pH** limits the diversity and number of microbes, some still persist, particularly in the mucus layer. *It establishes in the body only after the neonatal period* - **Colonization with normal flora** begins at birth, with exposure to maternal vaginal and fecal flora during passage through the birth canal. - Environmental exposure and feeding patterns further shape the microbial composition throughout the **neonatal period** and infancy. *The flora in the small bronchi is similar to that of the trachea* - The **lower respiratory tract**, including the small bronchi and alveoli, is generally considered **sterile or has very sparse flora** due to mucociliary clearance and immune defenses. - The **trachea** does have a transient flora, but it is distinct from and much richer than the extremely sparse flora, if any, found in the small bronchi.
Question 3: An 18-year-old man has facial and upper back lesions that have waxed and waned for the past 6 years. On physical examination, there are 0.3- to 0.9-cm comedones, erythematous papules, nodules, and pustules most numerous on the lower face and posterior upper trunk. Other family members have been affected by this condition at a similar age. The lesions worsen during a 5-day cruise to the Adriatic. Which of the following organisms is most likely to play a key role in the pathogenesis of these lesions?
- A. Propionibacterium acnes (Correct Answer)
- B. Herpes simplex virus type 1
- C. Group A β-hemolytic streptococcus
- D. Mycobacterium leprae
Explanation: ***Propionibacterium acnes*** (now *Cutibacterium acnes*) - The presence of **comedones, papules, nodules, and pustules** on the face and upper back in an 18-year-old is classic for **acne vulgaris**. - **_P. acnes_** is a commensal bacterium that proliferates in clogged hair follicles, contributing to inflammation and lesion formation in acne due to its lipolytic activity and immune-activating properties. *Herpes simplex virus type 1* - **HSV-1** typically causes **oral herpes (cold sores)** or **genital herpes**, characterized by painful vesicles and ulcers. - The described lesions (comedones, papules, nodules, pustules) are not characteristic of HSV-1 infection. *Group A β-hemolytic streptococcus* - **Group A Strep** causes infections like **pharyngitis (strep throat)**, **impetigo**, or **cellulitis**, which are typically acute and rapidly spreading. - Its presence is not associated with chronic, polymorphic lesions characteristic of acne. *Mycobacterium leprae* - **_M. leprae_** is the causative agent of **leprosy**, presenting with skin lesions, nerve damage, and other systemic effects. - The skin lesions of leprosy are typically macules, papules, or nodules with sensory loss, not the comedones and pustules seen in acne.
Question 4: An organism produces cutaneous disease (malignant pustule or eschar) at the site of inoculation in handlers of animal skins. Most likely organism is:
- A. Neisseria meningitidis
- B. Bacillus anthracis (Correct Answer)
- C. Pseudomonas aeruginosa
- D. Cryptococcus neoformans
Explanation: ***Bacillus anthracis*** - This description is classic for **cutaneous anthrax**, characterized by a **malignant pustule** or **eschar** that develops at the site of inoculation. - The context of handling **animal skins** (e.g., wool-sorter's disease) is a key epidemiological clue for _Bacillus anthracis_ infection. *Neisseria meningitidis* - Primarily causes **meningitis** and **meningococcemia**, involving a petechial or purpuric rash, not a single eschar or malignant pustule. - There is no direct association with handling animal skins. *Pseudomonas aeruginosa* - This bacterium is often associated with **opportunistic infections** in immunocompromised individuals, burn patients, or those with indwelling medical devices. - While it can cause skin lesions (e.g., **ecthyma gangrenosum**), these are distinct from the anthrax eschar and are not linked to animal skin exposure. *Cryptococcus neoformans* - A **fungus** that primarily causes **cryptococcal meningitis** or pulmonary infections, especially in immunocompromised individuals. - Skin manifestations, when they occur, are typically papules, nodules, or ulcers, not the classic **cutaneous anthrax eschar**.
Question 5: A farmer presents with a subcutaneous wound on his foot with discharge. Microscopy of a white granule from the wound shows Gram-positive filamentous rods. What is the most likely organism?
- A. Staphylococcus aureus
- B. Histoplasma
- C. Nocardia (Correct Answer)
- D. Sporothrix
Explanation: ***Nocardia*** - The presence of **white granules** in the discharge, along with **Gram-positive, filamentous rods**, is highly characteristic of *Nocardia* infection, often forming **sulfur granules** (though not always yellow). - *Nocardia* is a common soil bacterium, making it a likely pathogen in a **farmer with a subcutaneous wound** related to environmental exposure. *Staphylococcus aureus* - While *Staphylococcus aureus* can cause skin infections and abscesses, it presents as **Gram-positive cocci in clusters**, not filamentous rods. - It does not typically form **granules** in the discharge in the same manner as *Nocardia*. *Histoplasma* - *Histoplasma* is a **dimorphic fungus** that causes systemic infections, often acquired by inhaling spores. - It would appear as **yeast forms** in tissue or cultures, not Gram-positive filamentous rods, and is not typically associated with subcutaneous wounds forming granules. *Sporothrix* - *Sporothrix schenckii* causes **sporotrichosis**, characterized by a **subcutaneous nodule** that progresses along lymphatic channels. - It is a **dimorphic fungus** (yeast in tissue, mold in culture) and would not appear as Gram-positive filamentous rods on microscopy.
Question 6: "Isomorphic response" can be a feature of the following except
- A. Tinea (Correct Answer)
- B. Warts
- C. Molluscum contagiosum
- D. Psoriasis
Explanation: ***Tinea*** - The **isomorphic response (Koebner phenomenon)** refers to the development of new skin lesions in areas of trauma due to an immunological process. - This phenomenon is **not typically seen in tinea** (fungal infections). - While tinea can spread to new areas, this occurs through **direct fungal inoculation and contact spread**, not through the true Koebner mechanism. *Warts* - **Warts** caused by human papillomavirus (HPV) can exhibit the **isomorphic response**. - Trauma to the skin can lead to **viral inoculation** in that area, resulting in new wart formation along lines of trauma. - This is a well-recognized example of Koebner phenomenon in viral infections. *Molluscum contagiosum* - **Molluscum contagiosum** (poxvirus infection) can demonstrate the **isomorphic response**. - **Scratching or rubbing** can spread the virus to new areas through autoinoculation. - New lesions develop along the lines of trauma, consistent with Koebner phenomenon. *Psoriasis* - **Psoriasis** is the **classic and most well-known** condition exhibiting the isomorphic response or Koebner phenomenon. - New psoriatic plaques appear in areas of **skin injury** (scratches, cuts, burns, surgical incisions, friction). - Seen in approximately **25-50%** of psoriasis patients.
Question 7: Which of the following is a false statement about the respective fungal infections?
- A. Superficial layers of skin are involved
- B. Microsporium involves nail (Correct Answer)
- C. Candida albicans causes skin infection
- D. Epidermophyton doesn't involve hair
Explanation: ***Microsporium involves nail*** - This statement is considered **false** for exam purposes because *Microsporum* species are **NOT primary causes** of **tinea unguium** (onychomycosis). - *Microsporum* species primarily cause **tinea capitis** (scalp ringworm) and **tinea corporis** (body ringworm). While rare cases of nail involvement have been reported, it is clinically insignificant. - **Tinea unguium** (onychomycosis) is predominantly caused by ***Trichophyton rubrum*** and ***Trichophyton mentagrophytes***, with *Epidermophyton floccosum* and *Candida* species also playing roles. *Superficial layers of skin are involved* - This is a **true statement** because dermatophyte infections generally affect the **stratum corneum**, hair, and nails, which are all superficial keratinized tissues. - These fungi are unable to penetrate deeper viable tissue due to their inability to grow at core body temperature and the presence of inhibitory factors in serum. *Candida albicans causes skin infection* - This is a **true statement** as *Candida albicans* is a common cause of **cutaneous candidiasis**, manifesting as intertrigo, diaper rash, and paronychia. - It thrives in warm, moist environments and can infect skin folds, mucous membranes, and damaged skin. *Epidermophyton doesn't involve hair* - This is a **true statement** because *Epidermophyton floccosum* is unique among dermatophytes in that it primarily causes infections of the **skin** (tinea corporis, tinea cruris, tinea pedis) and **nails** (onychomycosis) but **does not infect hair follicles**. - Unlike *Trichophyton* and *Microsporum*, *Epidermophyton* lacks the enzymatic machinery to invade hair shafts.
Question 8: All of the following statements about Staphylococcus aureus are true, except:
- A. About 30% of general population is healthy nasal carriers.
- B. Methicillin resistance is chromosomally mediated.
- C. Epidermolysin and TSS toxin are superantigens.
- D. Most common source of infection is cross infection from infected people. (Correct Answer)
Explanation: ***Incorrect: Most common source of infection is cross-infection from infected people.*** - The most common source of *Staphylococcus aureus* infection, particularly in community-acquired cases, is from the patient's **own endogenous flora**, especially from the **nasal passages or skin**. - While cross-infection in healthcare settings (nosocomial transmission) is a significant issue, it is NOT the primary or most common source for the majority of *S. aureus* infections overall. - **Endogenous infection from colonized sites** is the predominant mode of acquisition. *Correct: About 30% of the general population is healthy nasal carriers.* - This statement is accurate; approximately **20-40%** (with an average often cited as 30%) of healthy individuals carry *Staphylococcus aureus* asymptomatically in their **anterior nares**. - This asymptomatic carriage is a significant reservoir for both self-infection and transmission to others. *Correct: Epidermolysin and TSS toxin are superantigens.* - This statement is correct. **Toxic Shock Syndrome Toxin-1 (TSST-1)** and the **exfoliative toxins (Epidermolysins)** are both classified as **superantigens**. - Superantigens bypass normal antigen processing and directly bind to MHC class II molecules and T-cell receptors, causing **widespread T-cell activation** (up to 20% of T cells) and massive cytokine release, leading to systemic symptoms like fever, rash, and shock. *Correct: Methicillin resistance is chromosomally mediated.* - This statement is true. Methicillin resistance in *Staphylococcus aureus* (MRSA) is mediated by the ***mecA* gene**, which encodes for an altered penicillin-binding protein (**PBP2a** or **PBP2'**). - The *mecA* gene is located on a mobile genetic element called the **staphylococcal cassette chromosome *mec* (SCCmec)**, which integrates into the bacterial **chromosome** (hence chromosomally mediated). - PBP2a has low affinity for β-lactam antibiotics, allowing cell wall synthesis to continue despite antibiotic presence.
Question 9: Botryomycosis is a ___ disease
- A. Viral
- B. Bacterial (Correct Answer)
- C. Parasitic
- D. Fungal
Explanation: ***Bacterial*** - **Botryomycosis** is primarily a **bacterial infection**, commonly caused by *Staphylococcus aureus* or, less frequently, by gram-negative bacteria like *Pseudomonas aeruginosa*. - It presents as chronic suppurative granulomatous inflammation characterized by the presence of **"grains" or "granules"** composed of bacterial microcolonies surrounded by hyaline material. *Viral* - **Viral infections** are caused by viruses and are typically characterized by intracellular replication and various cytopathic effects. - Botryomycosis does not involve viral pathogens; its pathogenesis is entirely distinct from viral diseases. *Parasitic* - **Parasitic diseases** are caused by parasites such as protozoa, helminths, or ectoparasites. - The clinical and pathological features of botryomycosis, including the distinct bacterial grains, do not align with parasitic infections. *Fungal* - Although it can superficially resemble **mycetoma (a fungal infection)** due to the presence of "grains," botryomycosis is not caused by fungi. - Mycetoma involves fungal organisms like *Madurella mycetomatis* or *Actinomadura madura*, which are distinctly different from the bacterial agents of botryomycosis.
Question 10: Subdural empyema is most commonly caused by:
- A. Staphylococcus aureus
- B. Streptococcus pneumoniae (Correct Answer)
- C. H. influenzae
- D. E. coli
Explanation: ***Streptococcus pneumoniae*** - **_Streptococcus species_**, including **_S. pneumoniae_**, aerobic and anaerobic streptococci, and S. milleri group, are the **most common causative organisms** of **subdural empyema**, accounting for **60-70% of cases**. - **Subdural empyema** most frequently arises from **paranasal sinusitis** (50-80% of cases) or **otitis media/mastoiditis**, infections typically caused by **streptococcal species**. - The polymicrobial nature of sinus and ear infections explains why **streptococci** predominate in subdural empyema. *Staphylococcus aureus* - **_Staphylococcus aureus_** is an important cause of **subdural empyema**, particularly in **post-neurosurgical cases** and following **penetrating head trauma**. - It accounts for approximately **10-20% of cases** but is **not the most common** overall pathogen. - When subdural empyema follows **surgery** or **direct inoculation**, S. aureus becomes more likely than in community-acquired cases. *H. influenzae* - **_Haemophilus influenzae_** was historically significant before widespread **Hib vaccination**. - Currently uncommon as a cause of **subdural empyema**, especially in vaccinated populations. - May still be seen in unvaccinated individuals or those with underlying immunodeficiency. *E. coli* - **_E. coli_** is a rare cause of **subdural empyema** in adults. - More relevant in **neonatal meningitis** and infections in **immunocompromised hosts**. - Gram-negative bacilli are generally uncommon in subdural empyema compared to gram-positive cocci.
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