Skin and Soft Tissue Infections — MCQs
What is the diagnosis of an umbilicated, pearly white, asymptomatic skin lesion?
A diabetic patient developed cellulitis due to S. aureus, which was found to be methicillin resistant on the antibiotic sensitivity testing. All of the following antibiotics will be appropriate except ?
A diabetic patient presents with sudden-onset perineal pain. On examination, foul-smelling discharge and necrotic tissue are noted. Which of the following is the most characteristic feature of this condition?
A 50-year-old diabetic presents with a foot ulcer. Which pathogen is most likely?
What is the first-line treatment for gas gangrene?
Which of the following conditions is caused by Staphylococcus aureus?
Cellulitis is characterized as:
A child presents with grouped vesicles on the lips. What is the bedside investigation that you would like to do?
A 50-year-old diabetic presents with a foot ulcer. Which pathogen is most likely?
What is the causative organism for the condition depicted in the image?
Skin and Soft Tissue Infections Indian Medical PG Practice Questions and MCQs
Question 1: What is the diagnosis of an umbilicated, pearly white, asymptomatic skin lesion?
- A. EBV
- B. HSV
- C. Molluscum contagiosum (Correct Answer)
- D. None of the options
Explanation: ***Molluscum contagiosum*** - This **viral skin infection** typically presents with **multiple, small (2-5 mm), firm, pearly, dome-shaped papules** that have a **central umbilication**. - The lesions are usually **asymptomatic**, as described, though they can occasionally be itchy or inflamed. - Caused by a **poxvirus** and is highly contagious through direct contact. *EBV* - **Epstein-Barr Virus (EBV)** is primarily associated with **infectious mononucleosis**, which presents with fever, sore throat, and lymphadenopathy, not umbilicated skin lesions. - EBV can cause oral hairy leukoplakia in immunocompromised individuals, which is a white lesion, but it is **not pearly, umbilicated, or dome-shaped**. *HSV* - **Herpes Simplex Virus (HSV)** causes lesions that are typically **grouped vesicles on an erythematous base** that evolve into erosions or ulcers. - HSV lesions are often **painful or itchy** and **do not appear as pearly, umbilicated papules**. *None of the options* - This is incorrect because **Molluscum contagiosum** perfectly matches the clinical description of umbilicated, pearly white, asymptomatic skin lesions. - The classic **central umbilication** is the pathognomonic feature that distinguishes molluscum from other viral skin infections.
Question 2: A diabetic patient developed cellulitis due to S. aureus, which was found to be methicillin resistant on the antibiotic sensitivity testing. All of the following antibiotics will be appropriate except ?
- A. Vancomycin
- B. Teicoplanin
- C. Linezolid
- D. Imipenem (Correct Answer)
Explanation: ***Imipenem*** - **Imipenem** is a carbapenem antibiotic that is effective against many Gram-positive and Gram-negative bacteria, but it is **not active against MRSA (methicillin-resistant *Staphylococcus aureus*)**. - MRSA strains are resistant to all beta-lactam antibiotics, including penicillins, cephalosporins, and carbapenems like imipenem, due to the presence of the **mecA gene** which encodes for an altered penicillin-binding protein (PBP2a). *Vancomycin* - **Vancomycin** is a glycopeptide antibiotic that is a primary choice for treating **MRSA infections**, including cellulitis. - It inhibits cell wall synthesis by binding to the D-Ala-D-Ala precursor, preventing cross-linking, and is specifically active against **Gram-positive bacteria**. *Teicoplanin* - **Teicoplanin** is another glycopeptide antibiotic, similar to vancomycin, and is also considered a suitable agent for treating **MRSA infections**. - It works by inhibiting bacterial cell wall synthesis and has a **longer half-life** than vancomycin, allowing for less frequent dosing. *Linezolid* - **Linezolid** is an oxazolidinone antibiotic known for its activity against **Gram-positive bacteria**, including **MRSA** and vancomycin-resistant enterococci (VRE). - It inhibits protein synthesis by binding to the 50S ribosomal subunit, preventing the formation of the initiation complex.
Question 3: A diabetic patient presents with sudden-onset perineal pain. On examination, foul-smelling discharge and necrotic tissue are noted. Which of the following is the most characteristic feature of this condition?
- A. Mixed aerobic and anaerobic infection (Correct Answer)
- B. Urinary diversion may be considered in severe cases
- C. Bilateral orchidectomy is not routinely required
- D. Anti-gas gangrene serum is indicated only in specific cases
Explanation: **Mixed aerobic and anaerobic infection** - Fournier's gangrene is a polymicrobial infection typically involving a **synergistic mixture of aerobic and anaerobic bacteria**. - This mixed infection contributes to the rapid progression and tissue destruction seen in this condition, leading to the **foul-smelling discharge** due to anaerobic metabolism. *Anti-gas gangrene serum is indicated only in specific cases.* - Anti-gas gangrene serum is specifically for **Clostridium perfringens** infections, which can cause gas gangrene but is usually a distinct clinical entity from Fournier's. - While Clostridium species can be present in Fournier's gangrene, it is not the sole causative agent, and **broader antimicrobial therapy** is the mainstay of treatment, not antitoxin serum. *Urinary diversion may be considered in severe cases.* - Urinary diversion, such as a **suprapubic catheter**, may be necessary when the urethra or perineum is extensively involved or to prevent ongoing contamination of the surgical site. - However, it's not a primary treatment for the infection itself but rather an **adjunctive measure** to manage complicated cases of Fournier's gangrene. *Bilateral orchidectomy is not routinely required.* - **Testicular involvement** in Fournier's gangrene is rare due to the separate blood supply of the testes. - **Orchidectomy** is only performed if the testes themselves are affected by necrosis, which is uncommon and occurs in critically severe cases; routine removal is not indicated.
Question 4: A 50-year-old diabetic presents with a foot ulcer. Which pathogen is most likely?
- A. Pseudomonas aeruginosa
- B. Escherichia coli
- C. Candida albicans
- D. Staphylococcus aureus (Correct Answer)
Explanation: ***Staphylococcus aureus*** - **_Staphylococcus aureus_** is the most common pathogen isolated from **diabetic foot ulcers** due to its prevalence on the skin and ability to infect compromised tissues. - Diabetic patients are particularly susceptible to **_S. aureus_** infections due to **impaired immune function** and **poor circulation**. *Pseudomonas aeruginosa* - While _Pseudomonas aeruginosa_ can cause foot infections, it is typically associated with **chronic, wet wounds** or those exposed to water, and is less common as a primary pathogen than _S. aureus_. - Infections by _Pseudomonas_ often result in a **greenish discharge** and a characteristic fruity odor, which are not mentioned here. *Escherichia coli* - **_Escherichia coli_** is primarily a cause of **urinary tract infections** and **gastrointestinal infections**. - While it can be found in polymicrobial wound infections, it is not the most likely single pathogen to initiate a diabetic foot ulcer infection. *Candida albicans* - **_Candida albicans_** is a **fungus** that can cause infections, particularly in immunocompromised individuals and in moist areas. - While **fungal infections** can complicate diabetic foot ulcers, it is not the primary bacterial pathogen typically responsible for the initial presentation of such ulcers.
Question 5: What is the first-line treatment for gas gangrene?
- A. Debridement & antibiotics (Correct Answer)
- B. Hyperbaric oxygen
- C. Polyvalent gas gangrene antitoxin
- D. Amputation
Explanation: ***Debridement & antibiotics*** - **Aggressive surgical debridement** to remove necrotic tissue and reduce bacterial load is the most critical initial step. - **Broad-spectrum antibiotics**, particularly penicillin G, are essential to target the causative *Clostridium perfringens* and prevent systemic spread. *Hyperbaric oxygen* - While **hyperbaric oxygen therapy** can be a useful adjunct by inhibiting bacterial growth and toxin production in anaerobic environments, it is not the *first-line* or sole treatment. - It should be used in conjunction with debridement and antibiotics, not as a standalone initial therapy. *Polyvalent gas gangrene antitoxin* - **Antitoxins** are generally not recommended due to their limited efficacy and potential for severe allergic reactions. - The primary treatment focuses on removing the source of infection and killing the bacteria, not neutralizing toxins alone. *Amputation* - **Amputation** is a drastic measure typically reserved for cases where the limb is irreversibly damaged, infection is uncontrollable by other means, or there is a threat to life. - It is not the initial treatment but may be necessary in advanced or complicated cases.
Question 6: Which of the following conditions is caused by Staphylococcus aureus?
- A. Corynebacterium minutissimum infection
- B. Haemophilus ducreyi infection
- C. Propionibacterium acnes infection
- D. Bullous impetigo (Correct Answer)
Explanation: ***Bullous impetigo*** - Bullous impetigo is a superficial skin infection characterized by **blisters (bullae)**, and is specifically caused by **Staphylococcus aureus** producing exfoliative toxins. - The toxins produced by *S. aureus* cause intraepidermal cleavage, leading to the formation of the characteristic **flaccid bullae**. *Corynebacterium minutissimum infection* - *Corynebacterium minutissimum* causes **erythrasma**, a chronic superficial skin infection characterized by well-demarcated reddish-brown patches, often in intertriginous areas. - It does not cause bullous impetigo and is typically diagnosed by its coral-red fluorescence under a **Wood's lamp**. *Haemophilus ducreyi infection* - *Haemophilus ducreyi* is the causative agent of **chancroid**, a sexually transmitted infection characterized by painful genital ulcers with a necrotic base and often accompanied by swollen, tender regional lymph nodes. - It is not associated with skin blistering or bullous impetigo. *Propionibacterium acnes infection* - *Propionibacterium acnes* (now *Cutibacterium acnes*) is a bacterium commonly implicated in **acne vulgaris**, contributing to inflammation and comedone formation within hair follicles. - It causes inflammatory lesions like papules, pustules, nodules, and cysts, rather than bullous lesions.
Question 7: Cellulitis is characterized as:
- A. Suppurative and invasive
- B. Nonsuppurative and non-invasive
- C. Nonsuppurative and invasive (Correct Answer)
- D. Suppurative and non-invasive
Explanation: ***Nonsuppurative and invasive*** - Cellulitis is considered **nonsuppurative** as it typically lacks macroscopic pus formation, distinguishing it from abscesses. - It is **invasive** because it involves the dermal and subcutaneous tissues, spreading through fascial planes. *Suppurative and invasive* - This description is more indicative of conditions like an **abscess**, which involves localized collections of pus. - While abscesses are invasive, cellulitis characteristically lacks the discrete pus collection. *Nonsuppurative and non-invasive* - Conditions that are nonsuppurative and non-invasive might include self-limiting skin rashes or superficial inflammatory processes. - Cellulitis involves deeper tissue infection, which inherently makes it invasive. *Suppurative and non-invasive* - A condition that is suppurative but non-invasive would be rare and contradictory, as pus formation often indicates a tissue response that is at least locally invasive. - Superficial pustules might be considered suppurative and relatively non-invasive, but cellulitis clearly extends beyond such superficial lesions.
Question 8: A child presents with grouped vesicles on the lips. What is the bedside investigation that you would like to do?
- A. Wood's lamp
- B. Slit skin smear
- C. Tzanck smear (Correct Answer)
- D. KOH
Explanation: ***Tzanck smear*** - A **Tzanck smear** is a rapid bedside test that can identify **multinucleated giant cells**, which are seen in herpes simplex virus infections. - The presence of **grouped vesicles on the lips** is highly suggestive of **herpes labialis** (HSV-1), which is primarily a **clinical diagnosis**. - Among the options provided, Tzanck smear is the only relevant bedside investigation, though it has **limited sensitivity and specificity** and **cannot distinguish between HSV and VZV**. - In modern practice, **PCR or direct immunofluorescence** are preferred when laboratory confirmation is needed, but Tzanck smear remains a low-cost option in resource-limited settings. *Wood's lamp* - A Wood's lamp uses **ultraviolet light** to detect certain fungal or bacterial infections by revealing characteristic fluorescence. - It is useful for conditions like **tinea capitis** (green fluorescence) and **erythrasma** (coral-red fluorescence), but has no role in diagnosing viral vesicular lesions. *Slit skin smear* - A **slit skin smear** is used to detect **acid-fast bacilli** in the diagnosis of **leprosy**. - It is not indicated for vesicular lesions and is irrelevant to herpes simplex infection. *KOH* - A **KOH (potassium hydroxide) mount** is used to diagnose **fungal infections** by dissolving keratinocytes and revealing fungal hyphae or spores. - It has no utility in diagnosing viral infections such as herpes simplex.
Question 9: A 50-year-old diabetic presents with a foot ulcer. Which pathogen is most likely?
- A. Pseudomonas aeruginosa
- B. Escherichia coli
- C. Staphylococcus aureus (Correct Answer)
- D. Candida albicans
Explanation: ***Staphylococcus aureus*** - **Staphylococcus aureus** is the most common bacterial pathogen isolated from **diabetic foot ulcers**, often due to compromised skin integrity and neuropathy. - It can cause a range of infections, from superficial cellulitis to deep tissue infections and osteomyelitis, common in diabetic foot. *Pseudomonas aeruginosa* - While *Pseudomonas aeruginosa* can infect foot ulcers, especially in patients with **previous antibiotic exposure** or **immersion injuries**, it is less common as a primary pathogen than *S. aureus*. - Infections by *Pseudomonas* often present with a **characteristic sweet, grape-like odor** and a green-blue exudate. *Escherichia coli* - *Escherichia coli* is generally associated with infections originating from the **gastrointestinal or genitourinary tracts** and is not a typical primary cause of foot ulcers. - Its presence in foot ulcers is rare and often suggests **polymicrobial infection** or fecal contamination. *Candida albicans* - *Candida albicans* is a **fungal pathogen** that can cause infections, particularly in immunocompromised individuals or those with chronic moist conditions between the toes. - While diabetics are prone to **fungal infections**, *Candida* is not a common primary cause of a deep foot ulcer; bacterial infections are far more prevalent.
Question 10: What is the causative organism for the condition depicted in the image?
- A. Staphylococci (Correct Answer)
- B. Candidal infection
- C. Streptococcus
- D. Actinomycetes
Explanation: ***Staphylococci*** - The image shows **impetigo** with **crusted lesions**, consistent with **_Staphylococcus aureus_** infection. - **Staphylococcus aureus** is a major causative organism of impetigo, particularly **bullous impetigo**, and commonly produces the characteristic **honey-colored crusts** seen in non-bullous forms as well. - This superficial bacterial skin infection is highly contagious and responds well to topical or systemic antibiotics. *Candidal infection* - **Candidal infections** (e.g., candidiasis) typically present as **erythematous patches** with satellite lesions, or white plaques in mucosal areas, which is not consistent with the image. - This fungal infection is often seen in immunocompromised individuals or in warm, moist skin folds, not as crusted superficial lesions. *Streptococcus* - While **_Streptococcus pyogenes_** can also cause impetigo (especially non-bullous impetigo), the clinical presentation in the image is most consistent with **staphylococcal infection**. - Streptococcal infections may present similarly but can also cause other conditions like cellulitis or erysipelas with distinct features. *Actinomycetes* - **Actinomycosis** is a rare, chronic bacterial infection that forms **abscesses and sinus tracts**, often with "sulfur granules," which is distinct from the superficial skin lesions shown. - This infection usually involves deeper tissues and presents as a chronic, indolent infection, unlike the acute superficial presentation of impetigo.
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