Bacterial Skin Infections — MCQs

Bacterial Skin Infections Indian Medical PG Practice Questions and MCQs

Question 31: Which of the following are primary pyodermas?

A. Impetigo contagiosa
B. Ecthyma
C. Furuncle
D. All of the above (Correct Answer)

Explanation: **Explanation:** Pyodermas are bacterial skin infections (primarily caused by *Staphylococcus aureus* or *Streptococcus pyogenes*) classified into two categories: **Primary** and **Secondary**. **Primary Pyodermas** occur on previously healthy skin, have a characteristic morphology, and are usually caused by a single organism. * **Impetigo contagiosa:** A superficial infection (subcorneal) characterized by "honey-colored" crusts. It is the most common primary pyoderma in children. * **Ecthyma:** A deeper form of impetigo that extends into the dermis, resulting in "punched-out" ulcers covered with thick crusts. It often heals with scarring. * **Furuncle (Boil):** A deep-seated infection of the hair follicle (folliculitis) that leads to abscess formation. Multiple coalescing furuncles form a **Carbuncle**. Since all three conditions listed are classic examples of infections arising *de novo* on intact skin, **Option D (All of the above)** is the correct answer. **Why other options are not "incorrect" but incomplete:** Options A, B, and C are all primary pyodermas, but selecting any single one would be incomplete as the question asks for "which of the following" and all listed entities fit the definition. **High-Yield Clinical Pearls for NEET-PG:** * **Secondary Pyodermas (Infective Dermatitis):** These occur on pre-existing skin lesions (e.g., infected scabies, infected eczema, or pediculosis). * **Impetigo Neonatorum:** A bullous variant of impetigo in newborns; it is a medical emergency due to the risk of Staphylococcal Scalded Skin Syndrome (SSSS). * **Causative Organism:** *S. aureus* is the most common cause of bullous impetigo and furuncles, while *S. pyogenes* is frequently associated with non-bullous impetigo and ecthyma. * **Treatment:** Topical Mupirocin is the drug of choice for localized lesions.

Question 32: Saint Antony's Fire is a description given for which of the following conditions?

A. Erysipelas (Correct Answer)
B. Impetigo
C. Scarlet fever
D. Streptococcal tonsillitis

Explanation: **Explanation:** **Erysipelas** is the correct answer. It is a superficial variant of cellulitis, primarily caused by **Group A Beta-hemolytic Streptococci (GABHS)**. It involves the upper dermis and superficial lymphatics. The term **"Saint Anthony’s Fire"** was historically attributed to this condition due to the characteristic bright red, fiery appearance of the skin and the intense burning sensation experienced by patients. **Why the other options are incorrect:** * **Impetigo:** This is a highly contagious superficial pyoderma (subcorneal) characterized by "honey-colored crusts." It does not present with the deep, fiery erythema seen in Erysipelas. * **Scarlet Fever:** While also caused by GABHS, it is a systemic toxemia characterized by a "sandpaper-like" punctate rash and a "strawberry tongue," rather than localized dermal inflammation. * **Streptococcal Tonsillitis:** This is an infection of the pharynx and tonsils. While it can precede skin manifestations (like Scarlet Fever), it is not associated with the "Saint Anthony’s Fire" eponym. **High-Yield Clinical Pearls for NEET-PG:** 1. **Clinical Hallmark:** Erysipelas is distinguished from cellulitis by its **sharply demarcated, raised borders** and a "peau d'orange" appearance. 2. **Common Site:** The lower limbs are most common, followed by the face (often in a butterfly distribution). 3. **Milian’s Ear Sign:** Since the ear lacks deep dermis, involvement of the pinna suggests Erysipelas rather than deeper cellulitis. 4. **Treatment of Choice:** Penicillin remains the mainstay of treatment.

Question 33: Lupus vulgaris is:

A. A generalized collagen disease
B. A local collagen disease of the face
C. A syphilitic lesion on the face
D. A tuberculosis lesion of the skin on the face (Correct Answer)

Explanation: **Explanation:** **Lupus vulgaris** is the most common clinical variant of **Cutaneous Tuberculosis** in adults. It is a chronic, progressive form of the disease that occurs in individuals with a high degree of cell-mediated immunity (tuberculin sensitivity) against *Mycobacterium tuberculosis*. **Why the correct answer is right:** Lupus vulgaris typically presents as reddish-brown "apple-jelly" nodules, most commonly on the **face and neck** (though it can occur elsewhere). It spreads peripherally while causing central scarring and atrophy. It is classified as a **secondary** form of cutaneous TB, usually arising from endogenous spread (via blood, lymph, or direct extension) from an underlying internal focus. **Why the other options are wrong:** * **Options A & B:** Despite the name "Lupus" (which means 'wolf' in Latin, referring to the destructive nature of the lesions), it is **not** a collagen vascular disease like Systemic Lupus Erythematosus (SLE). It is an infectious granulomatous disease, not a primary disorder of collagen. * **Option C:** Syphilis is caused by *Treponema pallidum*. While tertiary syphilis can cause destructive skin lesions (gummas), Lupus vulgaris is specifically caused by the tubercle bacillus. **NEET-PG High-Yield Pearls:** * **Clinical Sign:** The pathognomonic sign is the **"Apple-jelly nodule"** seen on **diascopy** (blanching the lesion with a glass slide). * **Histopathology:** Shows well-formed **tuberculoid granulomas** with minimal caseation necrosis. * **Complication:** The most dreaded long-term complication is the development of **Squamous Cell Carcinoma (Marjolin’s ulcer)** within the chronic scar. * **Treatment:** Standard Anti-Tubercular Therapy (ATT) for 6 months.

Question 34: Bacillary angiomatosis is caused by which organism?

A. Bartonella henselae (Correct Answer)
B. Bordetella pertussis
C. Rickettsia conorii
D. Aggregatibacter actinomycetemcomitans

Explanation: **Explanation:** **Bacillary Angiomatosis (BA)** is a vascular proliferative disease caused by Gram-negative opportunistic bacteria of the genus *Bartonella*. It primarily affects immunocompromised individuals, particularly those with advanced HIV/AIDS (CD4 count <200 cells/mm³). 1. **Why Option A is Correct:** The two primary causative agents of BA are ***Bartonella henselae*** (transmitted via cat scratches/fleas) and ***Bartonella quintana*** (transmitted via human body lice). These organisms induce the proliferation of small blood vessels, leading to characteristic red-to-purple papules or nodules that can mimic Kaposi Sarcoma. Histologically, it is characterized by lobular vascular proliferation with neutrophilic infiltrate and the presence of bacteria on **Warthin-Starry silver stain**. 2. **Why the other options are incorrect:** * **B. Bordetella pertussis:** The causative agent of Whooping Cough; it does not cause cutaneous vascular lesions. * **C. Rickettsia conorii:** Causes Boutonneuse fever (Mediterranean spotted fever), characterized by a "tache noire" (black eschar) at the bite site and a maculopapular rash, not angiomatous proliferation. * **D. Aggregatibacter actinomycetemcomitans:** A member of the HACEK group associated with aggressive periodontitis and infective endocarditis. **High-Yield Clinical Pearls for NEET-PG:** * **Differential Diagnosis:** Must be clinically distinguished from **Kaposi Sarcoma** (caused by HHV-8). While BA has a neutrophilic infiltrate, Kaposi Sarcoma shows a lymphocytic infiltrate and "slit-like" vascular spaces. * **Treatment of Choice:** **Erythromycin** or Doxycycline (unlike Kaposi Sarcoma, which requires ART and chemotherapy). * **Other Bartonella diseases:** *B. henselae* also causes Cat Scratch Disease (lymphadenopathy in immunocompetent hosts), while *B. quintana* causes Trench Fever.

Question 35: A single hypopigmented anesthetic patch with a satellite lesion on the forearm is likely diagnosed as which of the following?

A. Indeterminate leprosy
B. Tuberculoid leprosy (Correct Answer)
C. Neuritic leprosy
D. Lupus vulgaris

Explanation: **Explanation:** The clinical presentation of a **single hypopigmented, anesthetic patch** associated with **satellite lesions** is a classic hallmark of **Tuberculoid Leprosy (TT)**. In Tuberculoid Leprosy, the host possesses high cell-mediated immunity (CMI). This strong immune response localizes the infection to a single or very few lesions (usually 1–3). The hallmark features include well-defined margins, complete loss of sensation (anesthesia) due to nerve damage within the patch, and hair loss (alopecia). **Satellite lesions** (smaller macules or papules near the periphery of the main patch) are characteristic of the polar tuberculoid spectrum. **Analysis of Incorrect Options:** * **Indeterminate Leprosy:** This is the earliest stage. It typically presents as a single hypopigmented macule with *vague* margins and *partial* (not complete) sensory loss. Satellite lesions are not seen here. * **Neuritic Leprosy:** This form involves purely neurological symptoms (nerve thickening, pain, and sensory loss) in the distribution of a nerve, but it is characterized by the **absence** of any visible skin patches. * **Lupus Vulgaris:** This is a form of cutaneous tuberculosis. While it presents as a plaque, it is typically reddish-brown ("apple-jelly" nodules on diascopy) and is **not anesthetic**. **High-Yield Clinical Pearls for NEET-PG:** * **Ridley-Jopling Classification:** TT is the most stable form with the highest CMI. * **Nerve Involvement:** In TT, nerves are often thickened asymmetrically (e.g., ulnar or greater auricular nerve). * **Lepromin Test:** Strongly **positive** in Tuberculoid Leprosy (reflecting high CMI) and negative in Lepromatous Leprosy. * **Histopathology:** Shows well-formed non-caseating granulomas with rare or absent Acid-Fast Bacilli (paucibacillary).

Question 36: All of the following are true about bullous impetigo EXCEPT?

A. Caused by Staphylococcus aureus
B. Common in neonates and infants
C. Lesions commonly occur on the face
D. Lesions have an erythematous base (Correct Answer)

Explanation: **Explanation:** Bullous impetigo is a localized form of staphylococcal scalded skin syndrome (SSSS). The hallmark of this condition is the formation of large, flaccid bullae due to the action of **Exfoliative Toxin A** produced by *Staphylococcus aureus*. **Why Option D is the Correct Answer (The Exception):** Unlike non-bullous impetigo, the bullae in bullous impetigo typically arise on **normal-appearing skin** and lack a surrounding halo of erythema. When these bullae rupture, they leave behind a moist, red, "varnish-like" erosive surface, but the base is generally **not erythematous** in the initial stages. **Analysis of Other Options:** * **Option A:** It is caused exclusively by *Staphylococcus aureus* (Phage group II, type 71). This differs from non-bullous impetigo, which can be caused by both *S. aureus* and *S. pyogenes*. * **Option B:** It is most common in neonates (where it is known as Pemphigus neonatorum) and young children due to lack of immunity against the toxin and immature renal clearance. * **Option C:** While it can occur anywhere, the face is a very common site, along with intertriginous areas like the axilla and groin. **NEET-PG High-Yield Pearls:** * **Target Protein:** The exfoliative toxin cleaves **Desmoglein-1** (found in the upper epidermis), leading to subcorneal splitting. * **Nikolsky Sign:** Usually negative in bullous impetigo (localized), but positive in SSSS (generalized). * **Treatment:** Topical Mupirocin for localized lesions; oral anti-staphylococcal antibiotics (e.g., Dicloxacillin or Cephalexin) for extensive cases.

Question 37: All of the following skin infections are caused by Staphylococcus aureus, EXCEPT:

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

Explanation: **Explanation:** The primary distinction in bacterial skin infections lies in the causative organism and the depth of involvement. **1. Why Erysipelas is the correct answer:** Erysipelas is a superficial form of cellulitis characterized by sharp, well-demarcated borders and prominent lymphatic involvement. It is classically caused by **Group A Beta-hemolytic Streptococci (Streptococcus pyogenes)**, not *Staphylococcus aureus*. It typically affects the upper dermis and superficial lymphatics. **2. Why the other options are incorrect:** * **Folliculitis:** This is a superficial infection of the hair follicle. **Staphylococcus aureus** is the most common causative agent. * **Furuncle (Boil):** This is a deeper, necrotizing infection of the hair follicle and surrounding tissue, forming an abscess. It is almost exclusively caused by **Staphylococcus aureus**. * **Cellulitis:** While both *Streptococcus* and *Staphylococcus* can cause cellulitis, **Staphylococcus aureus** is a major cause, especially when associated with an open wound, trauma, or abscess. **3. Clinical Pearls for NEET-PG:** * **Erysipelas vs. Cellulitis:** Erysipelas has **raised, sharply defined edges** (superficial), whereas Cellulitis has **indistinct, flat borders** (deep dermis/subcutaneous fat). * **Staphylococcal Scalded Skin Syndrome (SSSS):** Caused by exfoliative toxins (A and B) from *S. aureus*; targets Desmoglein-1. * **Impetigo:** *S. aureus* is the most common cause of both bullous and non-bullous impetigo (though *S. pyogenes* also causes non-bullous). * **Ecthyma:** A deeper form of impetigo extending into the dermis, often caused by *Streptococcus*.

Question 38: Staphylococcal infection causes all the following diseases except:

A. Impetigo
B. Erysipelas (Correct Answer)
C. Ecthyma
D. Scalded skin syndrome

Explanation: **Explanation:** The core concept in this question is differentiating between infections caused by **Staphylococcus aureus** and **Streptococcus pyogenes** (Group A Streptococcus). **Why Erysipelas is the correct answer:** Erysipelas is a superficial form of cellulitis characterized by sharp, well-demarcated borders and intense erythema. It is classically caused by **Group A Beta-hemolytic Streptococci**. Unlike Staphylococcal infections, which tend to be localized and abscess-forming, Streptococcal infections often spread through tissue planes via enzymes like hyaluronidase. **Analysis of incorrect options:** * **Impetigo:** Can be caused by both *S. aureus* and *S. pyogenes*. However, **Bullous Impetigo** is exclusively caused by *S. aureus* (due to exfoliative toxins). * **Ecthyma:** This is a deeper, "punched-out" ulcerative form of impetigo. While it can be caused by Streptococcus, *S. aureus* is frequently isolated as a primary or co-infecting pathogen. * **Scalded Skin Syndrome (SSSS):** This is caused specifically by **Staphylococcal exfoliative toxins** (ETA and ETB) which target Desmoglein-1 in the stratum granulosum. **High-Yield Clinical Pearls for NEET-PG:** 1. **Golden Crust:** Pathognomonic for non-bullous impetigo (Staph or Strep). 2. **Nikolsky Sign:** Positive in Staphylococcal Scalded Skin Syndrome (SSSS) but negative in Bullous Impetigo. 3. **Depth of Infection:** Erysipelas involves the upper dermis and superficial lymphatics, whereas Cellulitis (often Staph) involves the deep dermis and subcutaneous fat. 4. **Toxin-Mediated Staph Diseases:** SSSS, Toxic Shock Syndrome (TSS), and Bullous Impetigo.

Question 39: Ecthyma gangrenosum is caused by which organism?

A. Fungal infection
B. Pseudomonas aeruginosa (Correct Answer)
C. Staphylococcus
D. Streptococcus

Explanation: **Explanation:** **Ecthyma gangrenosum (EG)** is a classic cutaneous manifestation of severe, invasive septicemia, most commonly caused by **Pseudomonas aeruginosa** (a Gram-negative rod). **Why Pseudomonas aeruginosa is correct:** The pathogenesis involves direct bacterial invasion of the media and adventitia of small blood vessels (perivascular invasion), leading to **necrotizing vasculitis**. This causes local infarction and ischemic necrosis of the skin. Clinically, it presents as a painless, erythematous macule that rapidly evolves into a hemorrhagic bulla and eventually transforms into a characteristic **punched-out ulcer with a central black necrotic eschar** surrounded by an erythematous halo. It typically occurs in immunocompromised patients (e.g., neutropenia, malignancy, or severe burns). **Why other options are incorrect:** * **Fungal infection:** While some fungi (like *Aspergillus* or *Mucor*) can cause similar angioinvasive necrotic lesions, the term "Ecthyma gangrenosum" is pathognomonic for Pseudomonas. * **Staphylococcus & Streptococcus:** These are the primary causes of **Ecthyma** (a deep form of impetigo). Unlike Ecthyma gangrenosum, common Ecthyma is a localized infection not usually associated with sepsis or necrotizing vasculitis. **High-Yield Clinical Pearls for NEET-PG:** * **Commonest Site:** Anogenital area and axilla. * **Patient Profile:** Classically seen in patients with **febrile neutropenia**. * **Diagnosis:** Confirmed by skin biopsy (showing vasculitis with bacteria) and blood cultures. * **Management:** Medical emergency requiring intravenous antipseudomonal antibiotics (e.g., Piperacillin-Tazobactam, Ceftazidime, or Aminoglycosides).

Question 40: Which of the following conditions present with draining sinuses?

A. Mycetoma and Lupus vulgaris
B. Mycetoma and Scrofula (Correct Answer)
C. Mycetoma and Pediculosis
D. Scrofula and Lupus vulgaris

Explanation: **Explanation:** The presence of **draining sinuses** (chronic tracks connecting deep-seated infections to the skin surface) is a hallmark of specific chronic granulomatous and suppurative conditions. **1. Why the Correct Answer is Right:** * **Mycetoma (Madura Foot):** This is a chronic granulomatous infection (fungal or bacterial) characterized by a clinical triad of **localized swelling, painless subcutaneous nodules, and multiple discharging sinuses** that extrude characteristic "grains." * **Scrofula (Scrofuloderma):** This is a form of cutaneous tuberculosis resulting from the breakdown of skin over an underlying tuberculous focus (usually a lymph node or bone). It presents as firm, painless nodules that ulcerate and form **chronic discharging sinus tracks** with watery or caseous discharge. **2. Why Other Options are Wrong:** * **Lupus Vulgaris:** This is the most common chronic form of cutaneous TB in previously sensitized individuals. It typically presents as "apple-jelly" nodules on diascopy and plaques, but it **does not** typically form draining sinuses. * **Pediculosis:** This is a superficial infestation by lice (head, body, or pubic). It presents with pruritus and excoriations, not deep-seated sinus formation. **Clinical Pearls for NEET-PG:** * **Actinomycetoma vs. Eumycetoma:** Actinomycetoma (bacterial) usually progresses faster and has more numerous sinuses compared to Eumycetoma (fungal). * **Other conditions with sinuses:** Actinomycosis (lumpy jaw with sulfur granules) and Hidradenitis Suppurativa (intertriginous areas). * **Scrofuloderma** is the most common cause of cold abscesses in the neck leading to sinus formation.

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

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free