Bacterial Skin Infections — MCQs

Bacterial Skin Infections Indian Medical PG Practice Questions and MCQs

Question 121: What is the drug of choice for MRSA skin infection?

A. Streptogramin
B. Vancomycin (Correct Answer)
C. Quinupristin
D. Linezolid

Explanation: **Explanation:** **Methicillin-resistant Staphylococcus aureus (MRSA)** is a major cause of skin and soft tissue infections (SSTIs). It is characterized by the presence of the **mecA gene**, which alters penicillin-binding proteins (PBP2a), rendering all beta-lactams (except 5th generation cephalosporins) ineffective. **Why Vancomycin is the Correct Answer:** **Vancomycin** (a glycopeptide) remains the traditional **Drug of Choice (DOC)** for serious systemic MRSA infections and complicated SSTIs. It works by inhibiting bacterial cell wall synthesis at the D-Ala-D-Ala terminus. In the context of NEET-PG, unless "oral" or "minor infection" is specified, Vancomycin is the gold standard for MRSA. **Analysis of Incorrect Options:** * **Linezolid (Option D):** While highly effective against MRSA and often used for skin infections due to its excellent tissue penetration, it is generally reserved as a second-line agent or for cases where IV access is difficult (due to 100% oral bioavailability). * **Quinupristin/Dalfopristin (Option C):** These are **Streptogramins (Option A)**. While they cover MRSA, they are primarily reserved for Vancomycin-resistant strains (VRSA) or Vancomycin-resistant Enterococcus (VRE) due to their significant side effect profile (e.g., infusion-site reactions, arthralgia). **High-Yield Clinical Pearls for NEET-PG:** * **Oral DOC for MRSA (Minor infections):** Cotrimoxazole, Clindamycin, or Doxycycline. * **Newer agents for MRSA:** Daptomycin (not used in pneumonia), Telavancin, and Ceftaroline (5th gen Cephalosporin). * **Mupirocin:** Topical drug of choice for MRSA colonization (nasal) and Impetigo. * **Red Man Syndrome:** A common side effect of Vancomycin due to histamine release (managed by slowing the infusion rate).

Question 122: A 12-year-old child presents with a painless neck swelling in the supraclavicular region, which started discharging after a few days. What is the most probable diagnosis?

A. Scrofuloderma (Correct Answer)
B. Actinomycosis
C. Botomycosis
D. Fungal mycetoma

Explanation: ### **Explanation** **Correct Answer: A. Scrofuloderma** **Scrofuloderma** (Tuberculosis colliquativa cutis) is a form of cutaneous tuberculosis that occurs due to the **contiguous spread** of infection from an underlying tuberculous focus, most commonly a **lymph node** (especially cervical or supraclavicular) or a bone/joint. * **Clinical Presentation:** It typically begins as a firm, painless, subcutaneous nodule that gradually softens (liquefaction) and breaks down to form an **undermined ulcer** or a **discharging sinus**. * **Key Feature:** The classic presentation is a painless neck swelling in a child or young adult that eventually discharges pus or caseous material, often healing with characteristic **puckered (bridged) scars**. --- ### **Why Other Options are Incorrect:** * **B. Actinomycosis:** While it presents with discharging sinuses, it typically involves the **cervicofacial region (mandible)**, is associated with "lumpy jaw," and the discharge contains characteristic **sulfur granules**. * **C. Botryomycosis:** This is a chronic granulomatous bacterial infection (usually *S. aureus*) that mimics fungal infections. It presents with nodules and sinuses, usually on the **extremities** following trauma, rather than isolated supraclavicular nodes. * **D. Fungal Mycetoma:** This presents as a triad of tumefaction (swelling), sinuses, and **grain discharge**, usually on the **foot** (Madura foot). It is rare in the supraclavicular region. --- ### **High-Yield Clinical Pearls for NEET-PG:** * **Most common site for Scrofuloderma:** Cervical/Supraclavicular lymph nodes. * **Lupus Vulgaris:** The most common clinical variant of cutaneous TB overall (presents with "apple-jelly nodules" on diascopy). * **Tuberculosis Verrucosa Cutis (TVC):** Known as "Prosector’s wart," it occurs due to exogenous inoculation in a previously sensitized individual. * **Puckered Scars:** A pathognomonic sign of healed scrofuloderma.

Question 123: Very numerous, symmetrically distributed, erythematous or copper colored shiny macules/papules are a feature of which condition?

A. Tuberculoid leprosy
B. Borderline borderline leprosy
C. Lepromatous leprosy (Correct Answer)
D. Borderline tuberculoid leprosy

Explanation: ### Explanation The clinical presentation described—numerous, symmetrically distributed, shiny, erythematous, or copper-colored lesions—is a hallmark of **Lepromatous Leprosy (LL)**. #### Why Lepromatous Leprosy (LL) is Correct: In LL, the patient has a **deficient cell-mediated immunity (CMI)** against *Mycobacterium leprae*. This allows for uncontrolled bacillary multiplication and hematogenous spread. Because the spread is systemic, the lesions are: * **Numerous and Symmetrical:** Distributed equally on both sides of the body. * **Morphology:** Macules or papules that are typically small, shiny (due to atrophy/stretching of the skin), and have a characteristic **copper-red or erythematous hue**. * **Sensation:** Unlike other forms, sensation is often **preserved** in early LL lesions because nerve damage occurs much later and is more diffuse. #### Why Other Options are Incorrect: * **Tuberculoid Leprosy (TT):** Characterized by high CMI. Lesions are **few (usually single)**, asymmetrical, well-defined, and **anesthetic** (complete loss of sensation). * **Borderline Tuberculoid (BT):** Lesions are few, asymmetrical, and show significant sensory loss. They often have "satellite lesions" near the main patch. * **Borderline Borderline (BB):** This is the most unstable form. Lesions are moderate in number and show characteristic **"punched-out" or "Swiss-cheese" appearance** with sloping inner borders and well-defined outer borders. #### NEET-PG High-Yield Pearls: * **Leonine Facies:** Advanced LL feature due to diffuse infiltration of the face, loss of eyebrows (**Madarosis**), and thickening of the nose/ears. * **Grenz Zone:** A clear sub-epidermal band seen on histology in LL, separating the epidermis from the dermal leproma. * **Bacteriological Index (BI):** Highest in LL (4+ to 6+), as the lesions are "Multibacillary." * **Lepromin Test:** Strongly **negative** in LL (due to lack of CMI) and positive in TT.

Question 124: Buruli ulcer is caused by which microorganism?

A. Mycobacterium ulcerans (Correct Answer)
B. Mycobacterium marinum
C. Aycobacterium kansasi
D. Mycobacterium fortuitum

Explanation: **Explanation:** **Mycobacterium ulcerans** is the causative agent of **Buruli ulcer**, the third most common mycobacterial disease in humans (after tuberculosis and leprosy). The pathogenesis is unique because *M. ulcerans* produces a potent necrotizing toxin called **Mycolactone**. This lipid toxin has immunosuppressive and analgesic properties, which explains why the resulting deep, undermined ulcers are characteristically **painless** despite extensive tissue destruction. **Analysis of Incorrect Options:** * **Mycobacterium marinum:** Causes **Fish Tank Granuloma** or Swimming Pool Granuloma. It typically presents as a localized granulomatous lesion or a sporotrichoid spread (nodules along lymphatic drainage) after exposure to contaminated water or fish tanks. * **Mycobacterium kansasii:** Primarily causes a **pulmonary disease** resembling tuberculosis. Cutaneous involvement is rare and usually occurs in immunocompromised patients. * **Mycobacterium fortuitum:** A member of the **Rapidly Growing Mycobacteria (RGM)** group. It is commonly associated with post-surgical infections, trauma, or contaminated cosmetic procedures (e.g., pedicures), leading to skin abscesses or chronic discharging sinuses. **High-Yield Clinical Pearls for NEET-PG:** * **Toxin:** Mycolactone (essential for virulence; causes cell death without an inflammatory response). * **Clinical Feature:** Starts as a painless nodule or plaque that breaks down into a large ulcer with **deeply undermined edges**. * **Epidemiology:** Most common in West and Central Africa, often near slow-moving water. * **Treatment:** WHO recommends a combination of **Rifampicin and Clarithromycin** (or Streptomycin) for 8 weeks.

Question 125: Leprosy affects all organs except which of the following?

A. Eyes
B. Nerves
C. Uterus (Correct Answer)
D. Ovary

Explanation: **Explanation:** Leprosy, caused by *Mycobacterium leprae*, is a chronic granulomatous disease that primarily targets cooler areas of the body. The fundamental principle governing its distribution is the **temperature sensitivity** of the organism; *M. leprae* thrives best at temperatures between **27°C and 30°C**. **Why Uterus is the Correct Answer:** The **Uterus** is located deep within the pelvic cavity, maintaining a core body temperature of approximately **37°C**. This high temperature is inhibitory to the growth and survival of *M. leprae*. Consequently, the uterus (along with the lungs, CNS, and pancreas) is classically considered "immune" or resistant to leprosy. **Analysis of Incorrect Options:** * **Eyes (A):** The anterior segment of the eye is cooler than the core body temperature. Leprosy frequently involves the eyes, leading to lagophthalmos, corneal ulcers, and iridocyclitis. * **Nerves (B):** Peripheral nerves are a primary target. The bacteria have a unique tropism for **Schwann cells**, leading to neuropathy and characteristic nerve thickening. * **Ovary (D):** While internal, the ovaries can occasionally be involved in lepromatous leprosy (LL) due to hematogenous spread, leading to oophoritis, though this is less common than testicular involvement. **NEET-PG High-Yield Pearls:** * **Organs Spared in Leprosy:** CNS (Brain and Spinal Cord), Lungs, Uterus, Pancreas, and the "Warm areas" of the skin (axilla, groin, midline of the back). * **Most Common Nerve Involved:** Ulnar nerve. * **Testicular Involvement:** Very common in LL; can lead to sterility and gynecomastia due to low scrotal temperature. * **Cardinal Signs:** Hypopigmented patches with sensory loss, thickened nerves, and positive skin smears for Acid-Fast Bacilli.

Question 126: Which of the following organisms is implicated in the causation of botryomycosis?

A. Staphylococcus aureus (Correct Answer)
B. Staphylococcus albus
C. Pseudomonas aeruginosa
D. Streptococcus pneumoniae

Explanation: **Explanation:** **Botryomycosis** is a rare, chronic granulomatous bacterial infection that clinically mimics fungal infections (like actinomycosis or eumycetoma). Despite the fungal-sounding name (*botrys* meaning bunch of grapes), it is caused by **true bacteria**. 1. **Why Staphylococcus aureus is correct:** * **Staphylococcus aureus** is the most common causative organism (implicated in ~40% of cases). * The pathogenesis involves a low-virulence bacterial strain and a host with an altered immune response. This leads to the formation of characteristic **"grains" or "sulfur granules"** in the tissue, which are actually bacterial colonies surrounded by an eosinophilic Splendore-Hoeppli material (an antigen-antibody complex). 2. **Why the other options are incorrect:** * **Staphylococcus albus (S. epidermidis):** While a common skin commensal, it is rarely a primary pathogen in botryomycosis unless there is significant immunosuppression or foreign body involvement. * **Pseudomonas aeruginosa:** Although Gram-negative bacteria (like *Pseudomonas* or *E. coli*) can occasionally cause botryomycosis, they are much less frequent than *S. aureus*. * **Streptococcus pneumoniae:** This organism typically causes respiratory infections or meningitis and is not a recognized cause of the chronic cutaneous granulomatous lesions seen in botryomycosis. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Presents as nodules, abscesses, and multiple discharging sinuses, usually on the extremities (Cutaneous) or lungs (Visceral). * **Histology:** Look for the **Splendore-Hoeppli phenomenon** (intense eosinophilic material surrounding the bacterial colonies). * **Diagnosis:** Unlike actinomycosis (which is caused by anaerobic/microaerophilic filamentous bacteria), botryomycosis is caused by **common non-filamentous bacteria**. * **Treatment:** Long-term antibiotics based on culture sensitivity and surgical debridement.

Question 127: All of the following are modes of transmission of leprosy except?

A. Breast milk
B. Insect bite
C. Transplacental spread (Correct Answer)
D. Droplet infection

Explanation: ### Explanation Leprosy, caused by *Mycobacterium leprae*, is a chronic infectious disease primarily affecting the skin and peripheral nerves. Understanding its transmission is crucial for NEET-PG, as it often involves distinguishing between proven routes and theoretical possibilities. **Why Transplacental Spread is the Correct Answer:** Congenital leprosy (transplacental transmission) is **not** a recognized mode of transmission. Studies have shown that even children born to mothers with highly infectious lepromatous leprosy are born free of the disease. The placenta acts as an effective barrier against *M. leprae*. Infection in infants usually occurs postnatally through close prolonged contact with the mother. **Analysis of Other Options:** * **Droplet Infection (Option D):** This is the **most common** and primary mode of transmission. Large numbers of bacilli are shed from the nasal mucosa of untreated multibacillary (MB) patients during sneezing or coughing. * **Breast Milk (Option A):** *M. leprae* has been identified in the breast milk of lactating mothers with lepromatous leprosy. While not the primary route, it is a documented potential mode of transmission. * **Insect Bites (Option B):** Mechanical transmission via arthropod vectors (like flies or mosquitoes) has been demonstrated in experimental settings, though its epidemiological significance in humans remains minor. **High-Yield Clinical Pearls for NEET-PG:** * **Incubation Period:** Average 3–5 years (longest among bacterial infections). * **Primary Site of Entry:** Respiratory tract (nasal mucosa) is the most common; skin-to-skin contact is secondary. * **Reservoirs:** Humans are the main reservoir, but **Nine-banded armadillos** are a known zoonotic source. * **Infectivity:** Leprosy is highly infectious but has low pathogenicity (most people have natural immunity).

Question 128: Which of the following is considered a tuberculide?

A. Lichen scrofulosorum (Correct Answer)
B. Lichen nitidus
C. Lichen aureus
D. All of the above

Explanation: **Explanation:** **Lichen scrofulosorum** is the correct answer because it is a classic example of a **tuberculide**. Tuberculides are a group of skin eruptions representing a hypersensitivity reaction to *Mycobacterium tuberculosis* in an individual with high immunity. Unlike primary cutaneous tuberculosis, tuberculides do not contain the bacilli (paucibacillary) and are characterized by a positive Mantoux test and a rapid response to anti-tubercular therapy (ATT). **Why the other options are incorrect:** * **Lichen nitidus:** This is a chronic inflammatory condition of unknown etiology characterized by tiny, skin-colored, flat-topped papules. It is not related to tuberculosis. * **Lichen aureus:** This is a subtype of Pigmented Purpuric Dermatosis (PPD) characterized by golden-yellow or rust-colored macules/papules, typically due to hemosiderin deposition. It is a vascular/inflammatory condition, not an infectious hypersensitivity. **High-Yield Clinical Pearls for NEET-PG:** * **Lichen scrofulosorum:** Presents as asymptomatic, follicular, skin-colored to yellowish-brown papules, usually on the trunk of children or young adults with systemic TB (often lymph node or bone TB). * **Other Tuberculides to remember:** 1. **Erythema Induratum (Bazin’s disease):** Most common tuberculide; presents as tender nodules on the calves (posterior leg) that may ulcerate. 2. **Papulonecrotic Tuberculide:** Symmetrical dusky-red papules on extensors that heal with "varioliform" (pitted) scarring. * **Key Distinction:** In tuberculides, the PCR for TB DNA may be positive, but cultures are almost always negative.

Question 129: A 15-year-old boy presented with multiple hypoaesthetic macules with sloping edges, seen on the face, trunk, and arms. Histopathologic examination of a lesion shows numerous bacilli and granulomatous infiltration. What is your diagnosis?

A. Tuberculoid leprosy
B. Lepromatous leprosy
C. Borderline leprosy (Correct Answer)
D. Indeterminate leprosy

Explanation: ### Explanation The diagnosis is **Borderline Leprosy** (specifically Borderline Tuberculoid or BT), based on the clinical and histopathological findings. **1. Why Borderline Leprosy is correct:** * **Clinical Presentation:** The presence of **multiple** lesions (more than 5) and **sloping edges** (a characteristic feature of borderline leprosy) points away from the polar forms. While Tuberculoid leprosy (TT) has well-defined punched-out edges, Borderline lesions often show "satellite lesions" or "sloping edges" as they transition toward the lepromatous end. * **Histopathology:** The presence of **numerous bacilli** (multibacillary) alongside **granulomatous infiltration** is the clincher. In pure Tuberculoid leprosy, bacilli are almost never found (paucibacillary), whereas in Lepromatous leprosy, there is no organized granuloma (only Macrophage/Virchow cells). Borderline leprosy represents the unstable immunological middle ground where both features can coexist. **2. Why other options are incorrect:** * **Tuberculoid Leprosy (TT):** Characterized by a single or very few (1–3) lesions with sharply defined edges and **absent bacilli** on microscopy due to high cell-mediated immunity (CMI). * **Lepromatous Leprosy (LL):** Presents with numerous symmetrical nodules or macules with **ill-defined edges**. Histology shows "Grenz zone" and "Gloe" (clumps of bacilli), but **not** well-formed granulomas. * **Indeterminate Leprosy:** This is the earliest stage. It presents as a single hypopigmented macule with vague sensory loss and **non-specific histology** (no granulomas). **3. NEET-PG High-Yield Pearls:** * **Sloping edges/Satellite lesions:** Pathognomonic for Borderline Tuberculoid (BT) leprosy. * **Punched-out/Clear-cut edges:** Characteristic of Tuberculoid (TT) leprosy. * **Inverted Saucer appearance:** Seen in Borderline Lepromatous (BL) leprosy. * **Swiss-cheese appearance:** Histological feature of Mid-borderline (BB) leprosy due to punched-out clear areas within the granuloma.

Question 130: What is the most clinically significant skin eruption associated with Mycoplasma pneumoniae infection?

A. Erythema multiforme major (Correct Answer)
B. Bullous eruptions
C. Toxic epidermal necrolysis
D. Pyostomatitis vegetans

Explanation: **Explanation:** *Mycoplasma pneumoniae* is the most common infectious trigger for **Erythema Multiforme (EM)**, particularly the **EM Major** subtype. While Herpes Simplex Virus (HSV) is the overall most common cause of EM Minor, *Mycoplasma* is classically associated with more severe presentations involving extensive mucosal involvement (oral, ocular, or genital) alongside characteristic target lesions. **Why the correct answer is right:** *Mycoplasma* induces an immune-mediated hypersensitivity reaction. In EM Major, patients present with the classic "triad" of targetoid skin lesions, systemic symptoms (fever, malaise), and involvement of at least two mucosal surfaces. Recently, a distinct entity called **MIRA** (*Mycoplasma pneumoniae*-induced rash and mucositis) has been described, which often presents with prominent mucositis and minimal or absent skin lesions. **Analysis of incorrect options:** * **B. Bullous eruptions:** While EM Major can have vesicles or bullae in the center of target lesions, "bullous eruptions" is a non-specific term covering many primary blistering diseases (like Pemphigus). * **C. Toxic Epidermal Necrolysis (TEN):** TEN is almost exclusively drug-induced (e.g., sulfonamides, NSAIDs). While *Mycoplasma* can rarely mimic SJS/TEN, EM Major remains the classic and most significant association. * **D. Pyostomatitis vegetans:** This is a rare cutaneous marker specifically associated with **Inflammatory Bowel Disease (IBD)**, particularly Ulcerative Colitis, characterized by "snail-track" pustules in the mouth. **High-Yield Clinical Pearls for NEET-PG:** * **Target Lesion (Iris lesion):** Pathognomonic for EM; consists of three zones (central dusky/blister, edematous pale ring, and outer erythematous halo). * **MIRA:** Suspect this in a pediatric/young adult patient with severe stomatitis and a history of atypical pneumonia. * **Treatment:** Addressing the underlying *Mycoplasma* infection with macrolides (Azithromycin) is essential alongside supportive care.

Practice by Chapter

Impetigo

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Folliculitis, Furuncles, and Carbuncles

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Ecthyma

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Erysipelas and Cellulitis

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Staphylococcal Scalded Skin Syndrome

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Necrotizing Fasciitis

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Cutaneous Tuberculosis

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Leprosy

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Lyme Disease

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Syphilis

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Antibiotic Resistance in Dermatology

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Prophylaxis and Management

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