Bacterial Skin Infections — MCQs

Bacterial Skin Infections Indian Medical PG Practice Questions and MCQs

Question 51: Impetigo contagiosa is caused by which of the following microorganisms?

A. Group A beta-hemolytic streptococci (Correct Answer)
B. Staphylococcus
C. Haemophilus influenzae
D. Pseudomonas

Explanation: **Explanation:** **Impetigo contagiosa** (also known as non-bullous impetigo) is the most common bacterial skin infection in children. It is primarily caused by **Group A beta-hemolytic streptococci (GABHS)**, specifically *Streptococcus pyogenes*. While *Staphylococcus aureus* is now frequently isolated from these lesions (often as a co-infection), GABHS remains the classic and historically definitive cause of the non-bullous variety characterized by "honey-colored" crusts. **Analysis of Options:** * **Option A (Correct):** GABHS is the primary pathogen for non-bullous impetigo. It typically starts as a vesicle or pustule that ruptures to form the pathognomonic golden-yellow crust. * **Option B (Incorrect):** While *Staphylococcus aureus* is the **sole** cause of **Bullous Impetigo** (mediated by exfoliative toxins), it is considered a secondary or co-infective agent in the classic contagiosa form in many textbooks, though modern epidemiology shows its rising prevalence. * **Option C (Incorrect):** *Haemophilus influenzae* is associated with cellulitis (specifically periorbital) in children, but not impetigo. * **Option D (Incorrect):** *Pseudomonas* is associated with "hot tub folliculitis" and ecthyma gangrenosum, not impetigo. **High-Yield Clinical Pearls for NEET-PG:** 1. **Pathognomonic Sign:** "Honey-colored" or "Golden-yellow" crusts on an erythematous base. 2. **Bullous vs. Non-bullous:** Bullous impetigo is always *S. aureus*; Non-bullous is *S. pyogenes* (GABHS) or *S. aureus*. 3. **Complication:** The most serious complication of Impetigo contagiosa is **Post-Streptococcal Glomerulonephritis (PSGN)**. Note that treating the skin infection does *not* necessarily prevent PSGN (unlike Rheumatic Fever). 4. **Treatment:** Topical Mupirocin is the drug of choice for localized lesions. Systemic antibiotics (e.g., Amoxicillin-Clavulanate) are used for widespread cases.

Question 52: Erythematous lesions seen in the axilla with coral red fluorescence under a Wood's lamp indicate which of the following conditions?

A. Tinea
B. Lichen planus
C. Erythrasma (Correct Answer)
D. Vitiligo

Explanation: **Explanation:** The clinical presentation of erythematous, well-demarcated patches in intertriginous areas (like the axilla) showing **coral red fluorescence** under a Wood’s lamp is pathognomonic for **Erythrasma**. **1. Why Erythrasma is correct:** Erythrasma is a superficial bacterial infection caused by ***Corynebacterium minutissimum***. This organism produces **porphyrins** (specifically Coproporphyrin III) as a metabolic byproduct. When exposed to ultraviolet light (Wood’s lamp), these porphyrins emit a characteristic coral red glow. **2. Why other options are incorrect:** * **Tinea (Dermatophytosis):** While it presents with erythematous patches, it typically shows central clearing (annular) and active borders. Under Wood's lamp, most Tinea corporis/cruris do not fluoresce; only specific species like *Microsporum* (Tinea capitis) show bright green fluorescence. * **Lichen Planus:** This is an inflammatory condition characterized by the "6 Ps" (Planar, Purple, Polygonal, Pruritic, Papules, and Plaques). It does not show specific fluorescence. * **Vitiligo:** This is a depigmenting disorder. Under Wood’s lamp, it shows **milky-white** fluorescence due to the complete loss of melanin, which enhances the reflection of light. **High-Yield Clinical Pearls for NEET-PG:** * **Treatment of Choice:** Topical or oral **Erythromycin** (or Clindamycin). * **Common Sites:** Axilla, groin (most common), and toe webs (4th-5th space). * **Differential Diagnosis:** Often confused with Tinea cruris or Intertrigo; Wood's lamp is the gold standard for rapid bedside differentiation. * **Microscopy:** KOH mount will be negative for fungi, but Gram stain shows Gram-positive filamentous bacilli.

Question 53: Satellite lesions are seen in which type of leprosy?

A. Tuberculoid Leprosy
B. Lepromatous Leprosy
C. Borderline Tuberculoid Leprosy (Correct Answer)
D. Histoid Leprosy

Explanation: **Explanation:** The presence of **satellite lesions** is a hallmark clinical feature of **Borderline Tuberculoid (BT) Leprosy**. In the Ridley-Jopling classification, BT leprosy represents a stage where the host's cell-mediated immunity (CMI) is significant but not as robust as in polar Tuberculoid leprosy. 1. **Why Borderline Tuberculoid (BT) is correct:** Satellite lesions are smaller, clinically similar lesions (macules or plaques) found in the immediate vicinity of a larger primary lesion. They indicate a degree of immunological instability and the beginning of hematogenous spread that the body is attempting to contain. 2. **Why other options are incorrect:** * **Tuberculoid Leprosy (TT):** Characterized by a single or very few (1–3) well-defined, anesthetic, hairless plaques. The CMI is very high, keeping the infection strictly localized; thus, satellite lesions are absent. * **Lepromatous Leprosy (LL):** Characterized by widespread, symmetrical, bilateral nodules or macules due to negligible CMI. The lesions are too numerous and generalized for "satellite" terminology to apply. * **Histoid Leprosy:** A variant of LL (often due to dapsone resistance) characterized by firm, shiny, hemispherical cutaneous nodules on normal-looking skin. It does not feature satellite lesions. **High-Yield Clinical Pearls for NEET-PG:** * **BT Leprosy** is the most common clinical type of leprosy seen in India. * **Nerve Involvement:** In BT, nerve involvement is asymmetrical and more extensive than in TT. * **Inverted Saucer Appearance:** Characteristic of **Borderline Borderline (BB)** leprosy. * **Swiss Cheese Appearance:** Histopathological feature of **Borderline Lepromatous (BL)** leprosy. * **Face:** If a lesion is on the face with satellite lesions, it is a strong indicator of BT leprosy.

Question 54: Which of the following conditions is characterized by 'honey-colored' crusts?

A. Nummular eczema
B. Impetigo (Correct Answer)
C. Herpes zoster
D. Cutaneous diphtheria

Explanation: **Explanation:** **Impetigo** is the correct answer. It is a highly contagious, superficial bacterial skin infection most commonly caused by *Staphylococcus aureus* or *Streptococcus pyogenes*. The hallmark clinical feature of **Non-bullous Impetigo** (the most common form) is the formation of thin-walled vesicles or pustules that rupture to release an exudate, which dries into characteristic **"honey-colored" (golden) crusts**. These are typically found on the face, particularly around the nose and mouth. **Why the other options are incorrect:** * **Nummular eczema:** Characterized by coin-shaped, itchy, erythematous plaques. While they can ooze, they do not typically form the classic golden crusts seen in impetigo. * **Herpes zoster:** Presents as painful, dermatomal, grouped vesicles on an erythematous base. The crusts formed after vesicles rupture are usually hemorrhagic or dark, not honey-colored. * **Cutaneous diphtheria:** Caused by *Corynebacterium diphtheriae*, it typically presents as a "punched-out" ulcer with a greyish-white **pseudomembrane**, rather than superficial golden crusting. **High-Yield Clinical Pearls for NEET-PG:** * **Bullous Impetigo:** Always caused by *S. aureus* producing exfoliative toxins (ETA, ETB) that target **Desmoglein 1**. * **Ecthyma:** A deeper form of impetigo that extends into the dermis, resulting in "punched-out" ulcers covered with thick crusts. * **Treatment:** Topical **Mupirocin** or Retapamulin is the first-line treatment for localized lesions. * **Complication:** While impetigo can lead to Post-Streptococcal Glomerulonephritis (PSGN), it does *not* lead to Rheumatic Fever.

Question 55: Which of the following is a common type of cutaneous tuberculosis?

A. Lupus vulgaris (Correct Answer)
B. Scrofuloderma
C. Tuberculous indurative B. verruca cutis
D. Erythema induratum

Explanation: **Explanation:** Cutaneous tuberculosis is classified based on the patient's immune status and the mode of infection (exogenous vs. endogenous). **Lupus Vulgaris (LV)** is the most common form of cutaneous tuberculosis worldwide. It is a chronic, progressive form occurring in individuals with a high degree of cell-mediated immunity. * **Lupus Vulgaris (Correct):** It typically presents as reddish-brown "apple-jelly" nodules on diascopy, most commonly on the face. It spreads by hematogenous, lymphatic, or contiguous routes from an internal focus. * **Scrofuloderma (Option B):** While common in India, it is the second most frequent type. It occurs due to direct extension from an underlying infected structure, such as a lymph node (most common) or bone. * **Tuberculous Verrucosa Cutis (Option C):** This is an exogenous reinfection form occurring in previously sensitized individuals with high immunity. It is often seen in pathologists or butchers ("prosector’s wart"). * **Erythema Induratum (Option D):** Also known as Bazin disease, this is a **tuberculid**—a hypersensitivity reaction to *M. tuberculosis* elsewhere in the body—rather than a direct infection of the skin. **High-Yield Clinical Pearls for NEET-PG:** * **Most common type (Global/India):** Lupus Vulgaris. * **Apple-jelly nodules:** Pathognomonic for Lupus Vulgaris (seen on diascopy). * **Most common site for Scrofuloderma:** Cervical lymph nodes. * **Lupus Vulgaris Risk:** Long-standing cases carry a risk of developing **Squamous Cell Carcinoma** (Marjolin’s ulcer). * **Diagnosis:** Histopathology shows well-formed epithelioid granulomas; however, AFB is often difficult to find (paucibacillary).

Question 56: Wood's lamp examination in dermatology typically reveals a coral red fluorescence in which of the following conditions?

A. Porphyria cutanea tarda
B. Erythrasma (Correct Answer)
C. Livedo reticularis
D. Hypomelanosis

Explanation: **Explanation:** **Erythrasma** is the correct answer. It is a superficial bacterial infection caused by *Corynebacterium minutissimum*. The hallmark diagnostic feature under **Wood’s lamp examination** is a characteristic **coral-red fluorescence**. This occurs because the bacteria produce **coproporphyrin III**, which fluoresces when exposed to ultraviolet light (365 nm). Clinically, it presents as well-demarcated, reddish-brown, macerated plaques in intertriginous areas (axilla, groin, or toe webs). **Analysis of Incorrect Options:** * **Porphyria Cutanea Tarda (PCT):** While PCT involves porphyrins, Wood’s lamp examination of the **urine** (not the skin) typically shows a pink-orange or coral-red fluorescence. Skin lesions themselves do not typically show this fluorescence. * **Livedo Reticularis:** This is a vascular condition characterized by a reticulated (net-like) cyanotic pattern due to altered blood flow. It does not exhibit fluorescence under Wood’s lamp. * **Hypomelanosis (e.g., Vitiligo):** Wood’s lamp is used here to detect pigment loss. Depigmented areas (Vitiligo) appear **milky-white** and accentuated, while hypopigmented areas (Pityriasis versicolor) may show a pale yellow/gold fluorescence. **High-Yield Clinical Pearls for NEET-PG:** * **Tinea Versicolor:** Shows **yellowish-gold** fluorescence (due to *Malassezia*). * **Tinea Capitis (Microsporum):** Shows **bright greenish** fluorescence. * **Pseudomonas (Burn infections):** Shows **apple-green** fluorescence (due to pyoverdin). * **Treatment of Erythrasma:** Topical fusidic acid or clindamycin; oral Erythromycin for extensive cases.

Question 57: What is the term for skin tuberculosis that involves the skin after the lymph nodes have been affected?

A. Lupus erythematosus
B. Lupus pernio
C. Scrofuloderma (Correct Answer)
D. None of the above

Explanation: **Explanation:** **Scrofuloderma** (also known as Tuberculosis Colliquativa Cutis) is the correct answer. It represents a **secondary form of cutaneous tuberculosis** that occurs due to the direct extension of an underlying tuberculous focus—most commonly a **tuberculous lymph node** (lymphadenitis), but occasionally an infected bone or joint—into the overlying skin. The pathogenesis involves the formation of a cold abscess that eventually breaks through the skin surface, leading to characteristic undermined ulcers and sinus tracts that heal with puckered (bridged) scarring. It is common in children and adolescents and is usually associated with a high degree of cell-mediated immunity (positive Mantoux test). **Analysis of Incorrect Options:** * **Lupus Erythematosus:** This is an autoimmune connective tissue disease (e.g., SLE or DLE), not an infectious process caused by *Mycobacterium tuberculosis*. * **Lupus Pernio:** This is a pathognomonic skin manifestation of **Sarcoidosis**, characterized by violaceous, indurated plaques on the nose, cheeks, and ears. It is not related to tuberculosis. **High-Yield Clinical Pearls for NEET-PG:** * **Lupus Vulgaris:** The most common clinical variant of cutaneous TB; characterized by "apple-jelly nodules" on diascopy. * **Tuberculosis Verrucosa Cutis (TVC):** Also known as "Prosector’s wart"; occurs due to exogenous inoculation in a previously sensitized individual. * **Erythema Induratum (Bazin’s Disease):** A form of tuberculid (hypersensitivity reaction) presenting as nodules on the posterior calves. * **Most common site for Scrofuloderma:** Cervical lymph nodes (neck).

Question 58: Scalded skin syndrome is caused by which microorganism?

A. Staphylococci (Correct Answer)
B. Pneumococci
C. Streptococci
D. Meningococci

Explanation: **Explanation:** **Staphylococcal Scalded Skin Syndrome (SSSS)**, also known as Ritter’s disease, is caused by **Staphylococcus aureus** (specifically phage group II, types 71 and 55). The underlying mechanism involves the release of **exfoliative toxins (ET-A and ET-B)**. These toxins act as "molecular scissors" that specifically target and cleave **Desmoglein-1**, a cell-adhesion molecule found in the desmosomes of the stratum granulosum. This leads to intraepidermal blistering and widespread denudation of the skin, mimicking a scald. **Analysis of Options:** * **A. Staphylococci (Correct):** As described, S. aureus produces the epidermolytic toxins responsible for the characteristic superficial skin cleavage. * **B. Pneumococci:** These primarily cause respiratory infections (pneumonia), meningitis, and otitis media; they do not produce exfoliative toxins. * **C. Streptococci:** While Group A Streptococcus (S. pyogenes) causes many skin infections (Impetigo, Erysipelas, Cellulitis), it does not cause SSSS. However, it is the primary cause of Scarlet Fever and Streptococcal Toxic Shock Syndrome. * **D. Meningococci:** These are associated with meningitis and meningococcemia, characterized by a petechial or purpuric rash (purpura fulminans), not skin scalding. **High-Yield Clinical Pearls for NEET-PG:** * **Nikolsky Sign:** Positive (gentle pressure on the skin causes the epidermis to shear off). * **Level of Cleavage:** Sub-corneal/Stratum granulosum (very superficial). * **Mucous Membranes:** Characteristically **spared** in SSSS (unlike Stevens-Johnson Syndrome/TEN, where they are severely involved). * **Diagnosis:** Usually clinical; however, the blister fluid in SSSS is typically **sterile** because the damage is toxin-mediated from a distant site (e.g., nasopharynx or conjunctiva). * **Treatment:** Intravenous antibiotics (e.g., Cloxacillin or Nafcillin) and supportive fluid care.

Question 59: A leprosy patient on multidrug therapy develops a type II lepra reaction. What is the recommended next course of action?

A. Stop multidrug therapy
B. Start steroids
C. Start steroids and clofazimine (Correct Answer)
D. None of the above

Explanation: **Explanation:** Type II Lepra Reaction (Erythema Nodosum Leprosum - ENL) is a **Type III hypersensitivity reaction** (immune-complex mediated) occurring primarily in lepromatous (LL) and borderline lepromatous (BL) leprosy. It is characterized by tender, evanescent subcutaneous nodules, fever, and systemic involvement (neuritis, arthritis, or orchitis). **Why Option C is Correct:** The management of ENL requires controlling the acute inflammation while preventing recurrences. 1. **Systemic Steroids (Prednisolone):** These are the first-line agents to rapidly suppress the acute inflammatory response and prevent nerve damage. 2. **Clofazimine:** This drug has both anti-mycobacterial and **anti-inflammatory** properties. In ENL, it is used in higher doses (up to 300 mg/day) to allow for the tapering of steroids and to prevent future episodes of ENL. **Why Other Options are Wrong:** * **Option A:** Multidrug Therapy (MDT) should **never** be stopped during a lepra reaction. Stopping MDT leads to drug resistance and disease progression. * **Option B:** While steroids are essential, using them alone (without clofazimine) often leads to "steroid dependence" or frequent relapses once the dose is tapered. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice:** For severe/recurrent ENL, **Thalidomide** is the most effective drug (though contraindicated in pregnancy due to phocomelia). * **Type I vs. Type II:** Type I is a Type IV hypersensitivity (Delayed) occurring in borderline cases; Type II is a Type III hypersensitivity (Immune-complex) occurring in multibacillary cases. * **MDT Protocol:** Always continue WHO-MDT during both Type I and Type II reactions.

Question 60: Which of the following is known to cause Pedal Botryomycosis?

A. Actinomyces somaliensis
B. Nocardia asteroides
C. Staphylococcus aureus (Correct Answer)
D. Candida glabrata

Explanation: **Explanation:** **Botryomycosis** is a rare, chronic granulomatous bacterial infection that clinically mimics deep fungal infections (mycetomas). Despite its name (which implies a fungal origin), it is a **bacterial infection** characterized by the formation of "grains" or "sulfur granules" within the tissues. 1. **Why Staphylococcus aureus is correct:** * **Staphylococcus aureus** is the most common causative agent of Botryomycosis (responsible for ~40% of cases). Other common bacteria include *Pseudomonas aeruginosa* and *E. coli*. * The pathogenesis involves a low-virulence strain of bacteria or a compromised host immune response, leading to a "Splendore-Hoeppli phenomenon"—an eosinophilic material surrounding the bacterial colonies, forming the characteristic grains. * **Pedal Botryomycosis** specifically refers to involvement of the foot, presenting with nodules, sinuses, and discharge containing grains, resembling a Madura foot. 2. **Analysis of Incorrect Options:** * **A & B (Actinomyces & Nocardia):** These are causes of **Actinomycetoma**. While they also produce grains and sinus tracts, they are filamentous bacteria (higher bacteria) and are distinct from the true bacterial botryomycosis. * **D (Candida glabrata):** This is a yeast. Botryomycosis is strictly a bacterial process; fungal causes of similar lesions are classified as **Eumycetoma**. **High-Yield Clinical Pearls for NEET-PG:** * **Key Histology:** Look for the **Splendore-Hoeppli phenomenon** (intense eosinophilic material around bacterial clusters). * **Differential Diagnosis:** Always differentiate from **Mycetoma**. In Botryomycosis, grains contain **cocci or bacilli**, whereas in Mycetoma, grains contain **fungal hyphae or filamentous bacteria**. * **Treatment:** Long-term antibiotics based on culture (usually targeting *S. aureus*).

Practice by Chapter

Impetigo

Practice Questions

Folliculitis, Furuncles, and Carbuncles

Practice Questions

Ecthyma

Practice Questions

Erysipelas and Cellulitis

Practice Questions

Staphylococcal Scalded Skin Syndrome

Practice Questions

Necrotizing Fasciitis

Practice Questions

Cutaneous Tuberculosis

Practice Questions

Leprosy

Practice Questions

Lyme Disease

Practice Questions

Syphilis

Practice Questions

Antibiotic Resistance in Dermatology

Practice Questions

Prophylaxis and Management

Practice Questions

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