Bacterial Skin Infections Practice Questions

Q: What is the commonest cause of cellulitis?

Streptococcus. **Explanation:** **Cellulitis** is an acute, spreading inflammation of the deep dermis and subcutaneous tissues. **Why Streptococcus is Correct:** The most common causative organism for cellulitis is **Group A Beta-hemolytic Streptococcus (Streptococcus pyogenes)**. Streptococci produce enzymes like hyaluronidase and streptokinase, which break down cellular components and allow the infection to spread rapidly through tissue planes. In children, *Streptococcus pneumoniae* may also be implicated. **Analysis of Incorrect Options:** * **Staphylococcus aureus (Option A):** While a common cause of skin infections, it is more typically associated with **localized, purulent** infections like abscesses, furuncles, and carbuncles. It is the most common cause of cellulitis only when there is an associated open wound, penetrating trauma, or abscess. * **E. coli (Option B):** This is a gram-negative organism. It is an uncommon cause of cellulitis, usually seen only in immunocompromised patients or those with specific exposures (e.g., surgical sites in the perineal area). * **Hemophilus influenzae (Option D):** Historically a common cause of facial and periorbital cellulitis in children, its incidence has significantly decreased due to the **HiB vaccine**. **High-Yield Clinical Pearls for NEET-PG:** 1. **Erysipelas vs. Cellulitis:** Erysipelas is a superficial form of cellulitis (upper dermis) with **sharply demarcated borders**, almost always caused by *Streptococcus*. 2. **Risk Factor:** Tinea pedis (fungal infection between toes) is the most common "portal of entry" for lower limb cellulitis. 3. **Special Scenarios:** * *Pasteurella multocida*: Cellulitis after dog/cat bites. * *Erysipelothrix rhusiopathiae*: "Erysipeloid" in butchers/fishermen. * *Vibrio vulnificus*: Cellulitis following exposure to salt water.

Q: Which of the following conditions is caused by Staphylococcus aureus?

Bullous impetigo. **Explanation:** **Bullous Impetigo** is the correct answer because it is a localized form of staphylococcal scalded skin syndrome. It is caused specifically by **Staphylococcus aureus** (Phage group II, type 71). The pathogenesis involves the production of **Exfoliative Toxin A**, which targets **Desmoglein-1**, a cell-adhesion molecule in the granular layer of the epidermis. This leads to subcorneal cleavage, resulting in large, flaccid, thin-walled bullae that rupture to leave behind a "collarette" of scale. **Analysis of Incorrect Options:** * **A. Erythrasma:** Caused by *Corynebacterium minutissimum*. It presents as well-demarcated, reddish-brown plaques in intertriginous areas and shows a characteristic **coral-red fluorescence** under Wood’s lamp. * **B. Chancroid:** A sexually transmitted infection caused by the Gram-negative coccobacillus ***Haemophilus ducreyi***. It is characterized by painful genital ulcers and "school of fish" appearance on microscopy. * **C. Acne vulgaris:** A multifactorial disease where the primary bacterium involved is ***Cutibacterium acnes*** (formerly *Propionibacterium acnes*), an anaerobe found in the sebaceous follicles. **High-Yield Clinical Pearls for NEET-PG:** 1. **Non-bullous Impetigo:** The most common form of impetigo; caused by *Staphylococcus aureus* (most common) or *Streptococcus pyogenes*. Characterized by **"honey-colored" crusts**. 2. **Staphylococcal Scalded Skin Syndrome (SSSS):** A systemic manifestation of the same exfoliative toxin; unlike bullous impetigo, the blisters in SSSS are sterile because the toxin spreads hematogenously from a distant site. 3. **Nikolsky Sign:** Usually positive in SSSS but negative in localized bullous impetigo.

Q: Which of the following structures is affected during the development of a furuncle?

Sebaceous gland. **Explanation:** A **furuncle** (commonly known as a boil) is a deep-seated, painful, inflammatory nodule that develops from a preceding folliculitis. It is almost always caused by *Staphylococcus aureus*. **Why the Sebaceous Gland is correct:** The pathophysiology of a furuncle involves the **pilosebaceous unit**. It starts as an infection of the hair follicle that extends deep into the dermis and involves the associated **sebaceous gland**. The infection leads to follicular abscess formation and perifollicular necrosis. Since the sebaceous gland is an integral anatomical component of the pilosebaceous unit, it is the primary structure affected during the development of the lesion. **Analysis of Incorrect Options:** * **A. Sweat gland:** Infections of the eccrine sweat glands are rare. Infection of the apocrine sweat glands (found in axillae/groin) is characteristic of *Hidradenitis suppurativa*, not a simple furuncle. * **C. Holocrine gland:** While a sebaceous gland *is* a type of holocrine gland (based on its method of secretion), in clinical dermatology and NEET-PG exams, the anatomical term **"Sebaceous gland"** is the specific and preferred answer for the site of a furuncle. **High-Yield Clinical Pearls for NEET-PG:** * **Carbuncle:** A cluster of interconnected furuncles that drain through multiple follicular openings. It most commonly occurs on the nape of the neck or back. * **Predisposing Factors:** Diabetes mellitus, obesity, and immunocompromised states. * **Danger Triangle of the Face:** Furuncles in this area (nose/upper lip) can lead to **Cavernous Sinus Thrombosis** via the facial and ophthalmic veins. * **Treatment:** Incision and drainage (I&D) is the mainstay for fluctuant lesions; systemic antibiotics (e.g., Cloxacillin) are used for associated cellulitis or systemic symptoms.

Q: A 19-year-old man develops a rash in the groin area. On examination, it is a large well-demarcated area of tan-brown discoloration around his left inguinal area. There is some scaling of the lesion when brushed with a tongue depressor. Which of the following is the most appropriate initial diagnostic test?

Potassium hydroxide (KOH) preparation of scrapings. ### Explanation The clinical presentation of a **well-demarcated, tan-brown, scaling lesion** in the inguinal area of a young male is highly suggestive of a superficial fungal infection, most commonly **Tinea cruris** (jock itch). **1. Why KOH Preparation is the Correct Answer:** The initial diagnostic step for any scaling skin lesion suspected of being fungal is a **Potassium Hydroxide (KOH) preparation**. KOH dissolves the keratinocytes, allowing for the clear visualization of fungal elements (septate hyphae and spores) under a microscope. It is a rapid, bedside, and cost-effective test that confirms the diagnosis before starting antifungal therapy. **2. Analysis of Incorrect Options:** * **Punch biopsy (A):** This is an invasive procedure reserved for atypical presentations, suspected malignancies, or inflammatory dermatoses that do not respond to treatment. It is not an initial test for a scaling groin rash. * **Tzanck smear (B):** This is used for the diagnosis of **herpetic infections** (HSV/VZV) to look for multinucleated giant cells. It is not used for fungal infections. * **Blood culture (D):** This is used for systemic or disseminated fungal infections (e.g., Candidemia). Superficial dermatophyte infections are localized to the stratum corneum and do not cause fungemia. **3. NEET-PG High-Yield Pearls:** * **Differential Diagnosis:** A major differential for this presentation is **Erythrasma** (caused by *Corynebacterium minutissimum*). While both appear brown and scaly, Erythrasma shows a characteristic **coral-red fluorescence** under a **Wood’s lamp**. * **Tinea Cruris vs. Candidiasis:** Tinea cruris typically **spares the scrotum**, whereas Inguinal Candidiasis often involves the scrotum and presents with **satellite lesions**. * **Clinical Sign:** The "scaling when brushed" mentioned in the prompt refers to the **"accentuation of scales"** seen in fungal infections.

Q: Globi is seen in which type of leprosy?

Lepromatous. ### Explanation **Correct Answer: B. Lepromatous** **Why it is correct:** In **Lepromatous Leprosy (LL)**, the patient exhibits a deficient cell-mediated immunity (CMI) against *Mycobacterium leprae*. This allows for the uncontrolled multiplication of bacilli within macrophages. These macrophages, heavily laden with bacilli and lipids, are termed **Virchow cells** (or foam cells). When these bacilli aggregate into dense, cigar-shaped bundles or large spherical masses held together by a waxy matrix (glis), they are called **Globi**. Globi are a hallmark of high bacterial load (multibacillary status) and are characteristically seen in LL and Borderline Lepromatous (BL) cases. **Why other options are incorrect:** * **A. Tuberculoid (TT):** In TT, the patient has strong CMI. This results in well-formed granulomas that effectively contain or eliminate the bacilli. Consequently, bacilli are rare or absent (paucibacillary), and Globi are never seen. * **D. Borderline Tuberculoid (BT):** Similar to TT, the CMI is relatively strong. While a few bacilli may be found, they do not aggregate into Globi. * **C. Borderline (BB):** While BB has more bacilli than the tuberculoid end, the classic formation of massive Globi is specifically the defining feature of the lepromatous pole (LL) where the Bacteriological Index (BI) is highest (5+ to 6+). **High-Yield Clinical Pearls for NEET-PG:** * **Fite-Faraco Stain:** The specific modification of the Ziehl-Neelsen stain used to identify *M. leprae* (which is less acid-fast than *M. tuberculosis*). * **Bacteriological Index (BI):** A logarithmic scale (0 to 6+) used to quantify density; Globi are typically seen when the BI is >4+. * **Grenz Zone:** A clear subepidermal band of uninvolved dermis seen in LL, separating the epidermis from the underlying lepromatous infiltrate. * **Lepromin Test:** Strongly positive in TT (strong CMI) and negative in LL (absent CMI).

Q: Which of the following statements about Bacillary angiomatosis is NOT true?

Aminoglycosides are useful in treatment. **Bacillary Angiomatosis (BA)** is a vascular proliferative disease caused by Gram-negative rickettsial organisms of the genus *Bartonella*, primarily occurring in immunocompromised individuals (especially those with advanced HIV/AIDS). ### **Explanation of Options:** * **Option C (Correct Answer):** Aminoglycosides are **not** the first-line treatment for BA. The drugs of choice are **Erythromycin** (Macrolides) or **Doxycycline** (Tetracyclines). Treatment is usually prolonged (3–4 months) to prevent relapse. * **Option A:** BA is caused by **Bartonella henselae** (transmitted via cat scratches/fleas) and **Bartonella quintana** (transmitted via human body lice). * **Option B:** *Bartonella* species can cause **Peliosis hepatis**, characterized by blood-filled cystic spaces in the liver, often presenting with hepatomegaly and elevated alkaline phosphatase. * **Option D:** In AIDS patients, systemic involvement is common. Beyond the skin and liver, it can involve the **brain** (causing encephalopathy), bones (osteolytic lesions), and lymph nodes. ### **High-Yield Clinical Pearls for NEET-PG:** * **Histopathology:** Shows lobular proliferation of capillaries with **plump "epithelioid" endothelial cells** and a neutrophilic infiltrate. * **Warthin-Starry Stain:** This silver stain is used to visualize the causative bacilli in tissue sections. * **Differential Diagnosis:** Clinically, BA closely resembles **Kaposi Sarcoma**. However, BA is characterized by a neutrophilic infiltrate, whereas Kaposi Sarcoma shows a lymphocytic infiltrate and spindle cells. * **Key Association:** *B. henselae* is also the causative agent of **Cat Scratch Disease** in immunocompetent hosts.

Q: A patient with HIV infection develops multibacillary leprosy. What is the recommended treatment?

Rifampicin, dapsone, and clofazimine.. The management of leprosy in HIV-positive patients follows the same principles as in HIV-negative patients. According to WHO guidelines and the National Leprosy Eradication Programme (NLEP), the presence of HIV co-infection does not alter the standard Multi-Drug Therapy (MDT) regimen. **Explanation of the Correct Answer:** * **Option B (Rifampicin, dapsone, and clofazimine):** This is the standard WHO MDT regimen for **Multibacillary (MB) leprosy**. It consists of Rifampicin (600 mg once monthly), Clofazimine (300 mg once monthly + 50 mg daily), and Dapsone (100 mg daily) for a duration of 12 months. HIV-infected individuals respond well to this standard therapy, and there is no evidence to suggest that the regimen should be modified or simplified. **Why Other Options are Incorrect:** * **Option A:** Withholding treatment is incorrect as leprosy is a progressive infectious disease that can lead to permanent nerve damage and disability, regardless of HIV status. * **Option C:** This is the regimen for **Paucibacillary (PB) leprosy**. Since the question specifies "multibacillary," clofazimine must be included to prevent drug resistance and ensure efficacy. * **Option D:** Monotherapy with Rifampicin is never recommended for leprosy due to the high risk of developing drug-resistant *Mycobacterium leprae*. **High-Yield Clinical Pearls for NEET-PG:** * **Immune Reconstitution Inflammatory Syndrome (IRIS):** In HIV patients, starting Antiretroviral Therapy (ART) may trigger "Leprosy-IRIS," often presenting as a sudden Type 1 (Reversal) Reaction. * **Drug Interactions:** Be cautious with **Rifampicin** as it is a potent enzyme inducer and can decrease the plasma levels of certain Protease Inhibitors (PIs) and NNRTIs used in HIV treatment. * **MB Leprosy Criteria:** Presence of >5 skin lesions, or >1 nerve involvement, or a positive skin smear at any site.

Q: Which type of leprosy is associated with Lucio's phenomenon?

Lepromatous leprosy. **Explanation:** **Lucio’s phenomenon** is a rare, severe necrotizing variant of Type 2 Lepra reaction. It occurs almost exclusively in patients with **Diffuse Non-nodular Lepromatous Leprosy** (also known as Lucio’s Leprosy), which is a subtype of **Lepromatous Leprosy (LL)**. 1. **Why Lepromatous Leprosy is correct:** The underlying mechanism involves a massive bacterial load (high Bacillary Index) leading to direct invasion of the vascular endothelium by *Mycobacterium leprae*. This causes an immune-complex mediated vasculitis, resulting in vascular occlusion, extensive skin infarction, and large, jagged, "punched-out" necrotic ulcers. It is most commonly seen in patients from Mexico and Central America. 2. **Why other options are incorrect:** * **Tuberculoid (TT) and Borderline Tuberculoid (BT):** These poles have high cell-mediated immunity and very low bacterial loads. They are associated with Type 1 reactions (reversal reactions), not necrotizing vasculitis. * **Borderline Leprosy (BB/BL):** While BL can present with Erythema Nodosum Leprosum (ENL), the specific clinical entity of Lucio’s phenomenon is restricted to the diffuse lepromatous form. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** Diffuse infiltration of skin (Madarosis/Lion-like face), lack of nodules, and painful necrotic ulcers. * **Histopathology:** Shows ischemic necrosis of the epidermis and dermis with heavy colonization of blood vessel walls by Acid-Fast Bacilli (AFB). * **Treatment:** Standard MDT (Multidrug Therapy) for Leprosy; unlike ENL, steroids are less effective, and the primary focus is wound care and systemic antibiotics.

Question 1

What is the commonest cause of cellulitis?

Accepted Answer

Streptococcus

Answer

Staphylococcus

Answer

E. coli

Answer

Hemophilus

Question 2

What is the most common cause of cellulitis of the face?

Accepted Answer

Pericoronitis

Answer

Trauma

Answer

Abscess

Answer

Bacterial infection

Question 3

Which of the following conditions is caused by Staphylococcus aureus?

Accepted Answer

Bullous impetigo

Answer

Erythrasma

Answer

Chancroid

Answer

Acne vulgaris

Question 4

Which of the following structures is affected during the development of a furuncle?

Accepted Answer

Sebaceous gland

Answer

Sweat gland

Answer

Holocrine gland

Answer

None of the above

Question 5

A 19-year-old man develops a rash in the groin area. On examination, it is a large well-demarcated area of tan-brown discoloration around his left inguinal area. There is some scaling of the lesion when brushed with a tongue depressor. Which of the following is the most appropriate initial diagnostic test?

Accepted Answer

Potassium hydroxide (KOH) preparation of scrapings

Answer

Punch biopsy of skin

Answer

Tzanck smear

Answer

Blood culture for fungi

Question 6

Globi is seen in which type of leprosy?

Accepted Answer

Lepromatous

Answer

Tuberculoid

Answer

Borderline

Answer

Borderline tuberculoid

Question 7

Which of the following statements about Bacillary angiomatosis is NOT true?

Accepted Answer

Aminoglycosides are useful in treatment

Answer

Caused by Bartonella henselae

Answer

Can cause Peliosis hepatis

Answer

The brain can be involved in AIDS patients

Question 8

In leprosy, which nerves are commonly involved?

Accepted Answer

High ulnar, low median

Answer

High median, low ulnar

Answer

Triple nerve palsy

Answer

High radial, low median

Question 9

A patient with HIV infection develops multibacillary leprosy. What is the recommended treatment?

Accepted Answer

Rifampicin, dapsone, and clofazimine.

Answer

No anti-leprosy drug should be given.

Answer

Rifampicin and dapsone.

Answer

Rifampicin only.

Question 10

Which type of leprosy is associated with Lucio's phenomenon?

Accepted Answer

Lepromatous leprosy

Answer

Tuberculoid leprosy

Answer

Borderline leprosy

Answer

Borderline tuberculoid leprosy

Bacterial Skin Infections — MCQs

Bacterial Skin Infections — MCQs

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