Bacterial Skin Infections — MCQs

Bacterial Skin Infections Indian Medical PG Practice Questions and MCQs

Question 11: Erysipelas is caused by which bacterium?

A. Staphylococcus aureus
B. Staphylococcus albus
C. Streptococcus pyogenes (Correct Answer)
D. Haemophilus

Explanation: **Explanation:** **Erysipelas** is a specific clinical variant of superficial cellulitis characterized by prominent lymphatic involvement. The correct answer is **Streptococcus pyogenes** (Group A Beta-hemolytic Streptococcus), which is the causative agent in the vast majority of cases. **Why Streptococcus pyogenes is correct:** Erysipelas involves the upper dermis and superficial lymphatics. *S. pyogenes* produces extracellular toxins and enzymes (like hyaluronidase) that allow it to spread rapidly through these superficial layers. Clinically, this manifests as a well-demarcated, raised, "fiery red" erythematous plaque, most commonly on the lower limbs or face (the "butterfly" distribution). **Analysis of Incorrect Options:** * **Staphylococcus aureus:** While it is the most common cause of *cellulitis* (which involves the deeper dermis and subcutaneous fat), it rarely causes classic erysipelas. *S. aureus* is more typically associated with localized, purulent infections like furuncles or impetigo. * **Staphylococcus albus:** Now known as *Staphylococcus epidermidis*, this is a commensal organism of the skin and rarely acts as a primary pathogen in healthy skin; it is more associated with prosthetic valve endocarditis or catheter infections. * **Haemophilus:** *Haemophilus influenzae* type B was historically a cause of facial cellulitis in children, but its incidence has plummeted due to the HiB vaccine. It does not cause classic erysipelas. **High-Yield Clinical Pearls for NEET-PG:** * **Milian’s Ear Sign:** Since the pinna lacks subcutaneous tissue, involvement of the ear indicates erysipelas (superficial) rather than cellulitis (deep). * **Distinguishing Feature:** The "sharp, well-demarcated border" is the hallmark of erysipelas, distinguishing it from the ill-defined borders of cellulitis. * **Treatment of Choice:** Penicillin remains the drug of choice due to the high sensitivity of *S. pyogenes*.

Question 12: An acute inflammation of the lymphatics of the skin or mucous membrane is known as:

A. Carbuncle.
B. Furuncle.
C. Cellulitis.
D. Erysipelas. (Correct Answer)

Explanation: **Explanation:** **Erysipelas** is the correct answer because it is defined as a superficial form of cellulitis that specifically involves the **superficial lymphatics** and the upper dermis. It is characterized by a sharp, well-demarcated, raised border and a bright red "St. Anthony’s Fire" appearance. It is most commonly caused by **Group A Beta-hemolytic Streptococci (GABHS)**. **Analysis of Incorrect Options:** * **Furuncle (Boil):** This is a deep-seated infection of a single **hair follicle**, usually caused by *Staphylococcus aureus*. It involves the follicle and the surrounding subcutaneous tissue, not the lymphatics. * **Carbuncle:** This is a cluster of interconnected furuncles (multiple infected hair follicles) that drain through several openings. It typically occurs in areas with thick skin, like the nape of the neck. * **Cellulitis:** While similar to erysipelas, cellulitis involves the **deeper dermis and subcutaneous fat**. Unlike erysipelas, its borders are ill-defined and not raised, and it does not primarily target the superficial lymphatics. **High-Yield Clinical Pearls for NEET-PG:** * **Milian’s Ear Sign:** Erysipelas can involve the pinna (ear) because it lacks subcutaneous tissue, whereas cellulitis cannot. * **Common Site:** The lower limbs are the most frequent site, followed by the face (malar area). * **Predisposing Factor:** Tinea pedis (fungal infection) often acts as a portal of entry for the bacteria. * **Treatment:** Penicillin is the drug of choice for Erysipelas due to the high sensitivity of Streptococci.

Question 13: Which of the following is NOT a characteristic feature of impetigo?

A. A superficial bacterial infection commonly due to Staphylococcus aureus.
B. Microscopic feature is the accumulation of lymphocytes beneath the stratum corneum. (Correct Answer)
C. Blister formation is due to bacterial production of a toxin that cleaves Desmoglein 1.
D. Typically involves exposed skin, mainly hands and face.

Explanation: ### Explanation **Why Option B is the Correct Answer (The Incorrect Statement):** Impetigo is a **neutrophilic** dermatosis, not lymphocytic. The hallmark microscopic feature of impetigo (specifically bullous impetigo and subcorneal pustular dermatoses) is the accumulation of **neutrophils** beneath the stratum corneum, forming a subcorneal pustule. Lymphocytic infiltrates are more characteristic of viral infections or chronic inflammatory conditions like lichen planus. **Analysis of Incorrect Options (Correct Statements):** * **Option A:** Impetigo is a highly contagious superficial infection. While *Streptococcus pyogenes* was historically common, **Staphylococcus aureus** is now the most frequent causative agent for both bullous and non-bullous forms. * **Option C:** In bullous impetigo, *S. aureus* (Phage group II) produces **Exfoliative Toxin A**. This toxin specifically targets and cleaves **Desmoglein 1** (a cell adhesion molecule), leading to acantholysis and blister formation in the granular layer. This is the same mechanism seen in Staphylococcal Scalded Skin Syndrome (SSSS). * **Option D:** Impetigo typically affects children and occurs on **exposed areas** such as the face (perioral and perinasal) and extremities, as these are most prone to minor trauma or insect bites. **High-Yield NEET-PG Pearls:** * **Non-bullous Impetigo:** Most common type; characterized by "honey-colored" (amber) crusts. * **Bullous Impetigo:** Characterized by flaccid bullae; always caused by *S. aureus*. * **Depth of Infection:** Impetigo is limited to the epidermis. If the infection extends deeper into the dermis, it is termed **Ecthyma** (punched-out ulcers). * **Complication:** While Impetigo can lead to **Post-Streptococcal Glomerulonephritis (PSGN)**, it does *not* cause Rheumatic Fever.

Question 14: On which day is the Lepromin test typically read?

A. 3rd day
B. 7th day
C. 2nd week
D. 4th week (Correct Answer)

Explanation: The **Lepromin test** is a skin test used to determine the cell-mediated immunity (CMI) of a patient against *Mycobacterium leprae*. It is not a diagnostic test for leprosy but is used for classification and prognosis. ### **Explanation of the Correct Answer** The Lepromin test consists of two distinct phases of reaction: 1. **Fernandez Reaction:** An early, non-specific delayed-type hypersensitivity reaction read at **48–72 hours**. 2. **Mitsuda Reaction:** A late, specific reaction read at **3 to 4 weeks (typically 21 days)**. The Mitsuda reaction is the clinically significant component used to classify leprosy. A positive Mitsuda reaction (induration >3mm) indicates strong CMI, typical of **Tuberculoid (TT)** leprosy, while a negative reaction indicates poor CMI, typical of **Lepromatous (LL)** leprosy. Therefore, the definitive reading is taken in the **4th week**. ### **Why Other Options are Incorrect** * **A (3rd day):** This corresponds to the **Fernandez reaction**. While it indicates previous exposure, it is often false-positive due to cross-reactivity with other mycobacteria and is not the primary reading for classification. * **B & C (7th day / 2nd week):** No significant clinical readings are standardized during these intervals for the Lepromin test. ### **High-Yield Clinical Pearls for NEET-PG** * **Antigen used:** Dharmendra lepromin (chloroform-extracted) or Mitsuda lepromin (autoclaved infected tissue). * **Diagnostic Value:** It is **NOT** used to diagnose leprosy (as healthy individuals in endemic areas can be positive). * **Prognostic Value:** It helps predict the stability of the disease; a positive test in a treated patient suggests a lower risk of relapse. * **Rule of Thumb:** TT is Lepromin (+), LL is Lepromin (-).

Question 15: What is the treatment of choice for erythrasma?

A. Doxycycline
B. Gentamicin
C. Clarithromycin (Correct Answer)
D. Moxifloxacin

Explanation: **Explanation:** **Erythrasma** is a superficial bacterial infection caused by ***Corynebacterium minutissimum***, a Gram-positive rod. It typically presents as well-demarcated, reddish-brown, macerated plaques in intertriginous areas (axilla, groin, and toe webs). 1. **Why Clarithromycin is correct:** While topical therapy (like Fusidic acid or Clindamycin) is effective for localized disease, **oral Clarithromycin (1g single dose)** or Erythromycin is considered the systemic treatment of choice. Macrolides effectively target the protein synthesis of *C. minutissimum*. Clarithromycin is often preferred over Erythromycin due to better patient compliance and fewer gastrointestinal side effects. 2. **Why other options are incorrect:** * **Doxycycline:** While it has activity against some Gram-positive bacteria, it is not the first-line agent for Erythrasma. It is more commonly used for acne, rosacea, or rickettsial infections. * **Gentamicin:** This is an aminoglycoside primarily used for Gram-negative aerobic infections. It is not effective against the causative agent of erythrasma. * **Moxifloxacin:** This is a broad-spectrum fluoroquinolone. While it might show efficacy, it is "overkill" and not the standard of care, reserved for more complex respiratory or skin/soft tissue infections. **High-Yield Clinical Pearls for NEET-PG:** * **Wood’s Lamp Examination:** The most characteristic diagnostic feature is **Coral-red fluorescence**, caused by the production of **Coproporphyrin III** by the bacteria. * **Differential Diagnosis:** Often confused with Tinea cruris (which has a central clearing and active border) or Intertrigo. * **KOH Mount:** Will be negative for fungal hyphae, helping rule out dermatophytosis.

Question 16: Lepra cells seen in leprosy are:

A. Lymphocytes
B. Plasma cells
C. Vacuolated Histiocytes (Correct Answer)
D. Neutrophils

Explanation: ### Explanation **Correct Answer: C. Vacuolated Histiocytes** In Leprosy (Hansen’s Disease), particularly in the **lepromatous (LL) pole**, there is a deficient cell-mediated immune (CMI) response. This allows *Mycobacterium leprae* to multiply uncontrollably within macrophages. These macrophages ingest the bacilli but are unable to digest them. As the bacilli multiply and undergo fatty degeneration, the cytoplasm of the macrophage becomes filled with lipids and clusters of bacilli (globi), giving it a characteristic **foamy or vacuolated appearance**. These modified macrophages are specifically termed **Lepra cells** or **Virchow cells**. **Analysis of Incorrect Options:** * **A. Lymphocytes:** While present in the granulomas of Tuberculoid Leprosy (TT) due to a strong CMI, they do not transform into the characteristic foamy Lepra cells. * **B. Plasma cells:** These are part of the chronic inflammatory infiltrate but are not the primary host cells for *M. leprae*. * **D. Neutrophils:** These are cells of acute inflammation. While they may be seen in Type 2 Lepra Reactions (Erythema Nodosum Leprosum), they are not the defining cellular feature of the disease pathology. **High-Yield Clinical Pearls for NEET-PG:** * **Grenz Zone:** A clear subepidermal zone of uninvolved dermis seen in Lepromatous Leprosy, separating the epidermis from the dermal infiltrate (Lepra cells). * **Globi:** Large clumps of acid-fast bacilli (AFB) found inside Lepra cells, often visualized using the **Fite-Faraco stain** (a modified Ziehl-Neelsen stain). * **Ridley-Jopling Classification:** Lepra cells are the hallmark of the Lepromatous (LL) end of the spectrum, where the Bacterial Index (BI) is highest. * **Histoid Leprosy:** A variant of LL characterized by spindle-shaped histiocytes resembling a dermatofibroma.

Question 17: Lucio reaction is seen in which of the following conditions?

A. Tuberculosis
B. Leprosy (Correct Answer)
C. Syphilis
D. Lymphogranuloma venereum

Explanation: **Explanation:** **Lucio reaction** (also known as *phenomenon of Lucio*) is a rare, severe, and life-threatening type of reactional state specifically seen in patients with **Diffuse Lepromatous Leprosy (DLL)**. It is most commonly associated with the non-nodular, "pure and primitive" diffuse form of leprosy (Lucio-Latapi leprosy), particularly in patients from Mexico and Central America. **Why Leprosy is correct:** The underlying mechanism is a **necrotizing vasculitis** caused by direct invasion of the endothelial cells of blood vessels by *Mycobacterium leprae*. This leads to vascular occlusion, tissue infarction, and subsequent large, jagged, "punched-out" necrotic ulcers. Unlike Type 2 Lepra reactions (Erythema Nodosum Leprosum), Lucio reaction does not typically present with systemic fever or constitutional symptoms initially, but focuses on cutaneous infarcts. **Why other options are incorrect:** * **Tuberculosis:** While TB can cause various skin manifestations (like Lupus Vulgaris or Scrofuloderma), it does not manifest as Lucio reaction. * **Syphilis:** Secondary syphilis presents with a generalized maculopapular rash (including palms and soles) or Condyloma lata, but not necrotizing vasculitis of this type. * **Lymphogranuloma venereum (LGV):** Caused by *Chlamydia trachomatis*, it primarily affects the lymphatics (buboes) and anogenital area, not the systemic vascular system. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice:** Standard WHO MDT for Leprosy; however, systemic corticosteroids and wound care are vital. Thalidomide is generally **not** effective for Lucio reaction (unlike ENL). * **Histopathology:** Shows massive infiltration of the dermis and blood vessel walls by Acid-Fast Bacilli (AFB). * **Triad of Lucio Leprosy:** Diffuse skin infiltration, alopecia of eyebrows/eyelashes, and the Lucio phenomenon.

Question 18: Which of the following is done for the quick diagnosis of Erythrasma?

A. Biopsy
B. KOH examination
C. Culture and sensitivity
D. Wood's lamp examination (Correct Answer)

Explanation: ### Explanation **Erythrasma** is a superficial bacterial infection caused by ***Corynebacterium minutissimum***, a Gram-positive bacterium. It typically presents as well-demarcated, reddish-brown, macerated, or scaly plaques in intertriginous areas (axilla, groin, and inframammary folds). **Why Wood’s Lamp is the Correct Answer:** The hallmark for a quick, bedside diagnosis of Erythrasma is **Wood’s lamp examination**. Under ultraviolet light (365 nm), the lesions exhibit a characteristic **coral-red fluorescence**. This occurs because *C. minutissimum* produces **coproporphyrin III**, which fluoresces when exposed to the Wood's lamp. This test is non-invasive, immediate, and highly specific. **Analysis of Incorrect Options:** * **A. Biopsy:** While histopathology would show bacteria in the stratum corneum, it is invasive, time-consuming, and unnecessary for such a distinct clinical presentation. * **B. KOH Examination:** This is used to diagnose fungal infections (Dermatophytosis or Candidiasis). While Erythrasma is a common differential for fungal intertrigo, KOH would be negative for hyphae/spores in Erythrasma. * **C. Culture and Sensitivity:** *C. minutissimum* requires special media (e.g., tissue culture medium with fetal bovine serum) and takes days to grow. It is not a "quick" diagnostic method. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Often confused with Tinea Cruris; however, Erythrasma lacks a central clearing and an active scaling border. * **Treatment of Choice:** Topical **Clindamycin** or **Fusidic acid** for localized cases; Oral **Erythromycin** or **Clarithromycin** for extensive involvement. * **Wood’s Lamp Summary:** * **Erythrasma:** Coral-red * **Tinea Versicolor:** Yellowish-white/Copper-orange * **Tinea Capitis (Microsporum):** Bright Green * **Pseudomonas:** Yellow-green (Pyoverdin) * **Vitiligo:** Milky-white (Depigmentation)

Question 19: Which of the following is NOT true of erysipelas?

A. It is typically caused by streptococcal infection.
B. It is contagious and infectious. (Correct Answer)
C. The margins are raised and sharply demarcated.
D. All of the above are true.

Explanation: ### Explanation **Erysipelas** is a superficial variant of cellulitis characterized by prominent lymphatic involvement. **Why Option B is the correct answer (The False Statement):** While erysipelas is an **infectious** condition (caused by a pathogen), it is **not considered contagious** in the clinical sense. It involves the deep epidermis and underlying dermis; the bacteria are not typically present on the skin surface in sufficient quantities to spread through casual contact. Unlike impetigo, which is highly contagious due to superficial crusts, erysipelas does not pose a significant risk of transmission to others. **Analysis of other options:** * **Option A (True):** The most common causative organism is **Group A Beta-hemolytic Streptococci** (*Streptococcus pyogenes*). While *Staph. aureus* can occasionally be involved, Streptococci remain the primary etiology. * **Option C (True):** This is the classic clinical hallmark. Because the infection is superficial, the edema creates **sharply demarcated, raised, and indurated borders** (Milian’s ear sign is a classic example, as the ear lacks deep subcutaneous tissue, making erysipelas more likely than cellulitis). **NEET-PG High-Yield Pearls:** * **Anatomical Level:** Erysipelas affects the **upper dermis** and superficial lymphatics, whereas cellulitis affects the deep dermis and subcutaneous fat. * **Clinical Appearance:** Characterized by a "shiny, orange-peel" texture (**Peau d'orange**) due to lymphatic obstruction. * **Common Site:** Historically the face (butterfly distribution), but currently, the **lower limbs** are the most frequent site. * **Treatment of Choice:** Systemic antibiotics, typically **Penicillin**, as it remains highly effective against Streptococci.

Question 20: A 12-year-old boy presents with a white facial lesion that has grown noticeably large. Examination reveals hypoesthesia over the lesion and a smooth surface. What is the likely diagnosis?

A. Lepromatous leprosy
B. Pityriasis alba
C. Asbestosis
D. Indeterminate leprosy (Correct Answer)

Explanation: **Explanation:** The clinical presentation of a **hypopigmented (white) lesion** with **hypoesthesia** (loss of sensation) in a young patient is a classic hallmark of **Indeterminate Leprosy**. 1. **Why Indeterminate Leprosy is correct:** This is the earliest clinical stage of leprosy. It typically presents as a single, ill-defined, hypopigmented macule. The crucial diagnostic feature is the **impairment of sensation** (temperature or touch) and/or decreased sweating within the patch, caused by early nerve involvement. In children, the face is a common site. Histopathology usually shows a non-specific lymphocytic infiltration around nerves and appendages. 2. **Why other options are incorrect:** * **Lepromatous Leprosy (LL):** This is the multibacillary, polar end of the spectrum. It presents with multiple, symmetrical, shiny nodules or plaques and widespread infiltration (Leonine facies). Sensation is usually preserved in early LL lesions. * **Pityriasis Alba:** A common pediatric condition (often associated with atopy) presenting as ill-defined hypopigmented patches with fine scaling. Crucially, there is **no sensory loss**. * **Asbestosis:** This is a restrictive lung disease caused by asbestos fiber inhalation; it has no relevance to hypopigmented skin lesions. **NEET-PG High-Yield Pearls:** * **Cardinal Signs of Leprosy (WHO):** 1. Hypopigmented/reddish patch with definite loss of sensation. 2. Thickened peripheral nerves. 3. Positive skin smear for Acid Fast Bacilli. * **Indeterminate Leprosy** often heals spontaneously or evolves into other types (Tuberculoid or Lepromatous) depending on the host's cell-mediated immunity (CMI). * **Differential for Hypopigmented patches:** If sensation is lost, think Leprosy. If sensation is intact, consider Pityriasis alba, Vitiligo, or Tinea versicolor.

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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