Bacterial Skin Infections — MCQs

Bacterial Skin Infections Indian Medical PG Practice Questions and MCQs

Question 81: Leprosy does NOT affect the axilla, groin, and scalp because:

A. These are warmer regions of the body (Correct Answer)
B. These are colder regions of the body
C. These areas have less blood supply
D. These areas have less sensory nerve supply

Explanation: **Explanation:** The causative organism of Leprosy, ***Mycobacterium leprae***, is an obligate intracellular bacterium with a unique temperature sensitivity. It thrives optimally at temperatures slightly lower than the core body temperature, specifically between **30°C and 33°C**. **1. Why Option A is correct:** The axilla, groin, and scalp are considered "warm zones" of the body because they are either well-insulated by skin folds (intertriginous areas) or have high vascularity and hair cover, maintaining a temperature close to the core body temperature (37°C). Because *M. leprae* cannot replicate efficiently at these higher temperatures, these areas are typically spared from clinical lesions. Conversely, the bacteria preferentially affect cooler areas like the nose, ears, and extensor surfaces of the limbs. **2. Why other options are incorrect:** * **Option B:** These areas are physiologically warmer, not colder. Colder regions (like the pinna of the ear) are actually the most common sites for finding the bacilli. * **Option C:** The scalp and groin are actually highly vascular areas. Blood supply does not dictate the distribution of leprosy; temperature does. * **Option D:** These regions have a rich sensory nerve supply. While leprosy is a disease of the nerves, the distribution of lesions is determined by the thermal requirements of the bacteria, not the density of the nerves. **NEET-PG High-Yield Pearls:** * **"Immune Privileged" Sites in Leprosy:** The midline of the back, axilla, groin, and scalp are often referred to as "spared areas." * **Diagnosis:** The **Ear lobe snip** is a classic site for Slit Skin Smear (SSS) because it is one of the coolest peripheral parts of the body. * **Target Organs:** *M. leprae* has a predilection for **peripheral nerves** (Schwann cells) and the **skin**.

Question 82: A patient develops honey-colored pustules on the skin. What is the most likely diagnosis?

A. Impetigo (Correct Answer)
B. Staphylococcal scalded skin syndrome
C. Carbuncle
D. Sycosis barbae

Explanation: **Explanation:** The clinical presentation of **honey-colored crusts** (or pustules that rupture to form them) is the classic hallmark of **Impetigo**, specifically the non-bullous variant. 1. **Why Impetigo is correct:** Impetigo is a highly contagious superficial bacterial infection primarily caused by *Staphylococcus aureus* or *Streptococcus pyogenes*. The "honey-colored" appearance occurs when serous fluid from ruptured vesicles or pustules dries on the skin surface. It typically affects the face (perioral and perinasal areas) and extremities. 2. **Why other options are incorrect:** * **Staphylococcal Scalded Skin Syndrome (SSSS):** This is a systemic toxin-mediated condition characterized by widespread erythema and large, flaccid bullae that lead to skin peeling (positive Nikolsky sign), rather than localized honey-colored crusts. * **Carbuncle:** This is a deeper infection involving a cluster of interconnected furuncles (boils) that extend into the subcutaneous fat, presenting as a painful, swollen, and discharging inflammatory mass. * **Sycosis barbae:** This is a deep folliculitis of the beard area. While it involves pustules, it is specifically localized to hair follicles and does not typically present with the characteristic honey-colored crusting seen in impetigo. **High-Yield Clinical Pearls for NEET-PG:** * **Non-bullous Impetigo:** Most common form; caused by both *S. aureus* and *S. pyogenes*. * **Bullous Impetigo:** Always caused by *S. aureus* (phage group II) producing **exfoliative toxin A**, which targets **Desmoglein-1**. * **Treatment:** Topical **Mupirocin** is the drug of choice for localized lesions. Systemic antibiotics (e.g., Amoxicillin-Clavulanate) are used for extensive cases. * **Complication:** Post-streptococcal glomerulonephritis (PSGN) can follow impetigo, but Rheumatic Fever does not.

Question 83: Which mycobacterium exclusively affects the skin?

A. Mycobacterium marinum (Correct Answer)
B. Mycobacterium fortuitum
C. Myobacterium xenopi
D. Mycobacterium chelonei

Explanation: **Explanation:** **Mycobacterium marinum** is the correct answer because it is a photochromogenic atypical mycobacterium that grows optimally at **30–32°C**. This lower temperature requirement prevents it from causing systemic infections in the warmer internal organs of the human body (37°C), thus restricting its clinical manifestation **exclusively to the skin** and subcutaneous tissues. It typically causes "Fish Tank Granuloma" or "Swimming Pool Granuloma," often presenting as a sporotrichoid spread (linear nodules along lymphatics) following trauma in aquatic environments. **Analysis of Incorrect Options:** * **Mycobacterium fortuitum & Mycobacterium chelonae:** These are Rapidly Growing Mycobacteria (RGM). While they frequently cause skin and soft tissue infections (especially post-surgical or post-injection abscesses), they are also known to cause systemic diseases, including osteomyelitis, endocarditis, and disseminated infections in immunocompromised hosts. * **Mycobacterium xenopi:** This is a slow-growing mycobacterium primarily associated with **pulmonary infections**, often mimicking tuberculosis, especially in patients with pre-existing lung disease. It is not restricted to the skin. **High-Yield Clinical Pearls for NEET-PG:** * **Temperature Sensitivity:** *M. marinum* and *M. ulcerans* (which causes Buruli ulcer) both prefer cooler temperatures, explaining their predilection for the skin. * **Sporotrichoid Spread:** Differential diagnoses include *Sporothrix schenckii*, *M. marinum*, *Nocardia*, and *Leishmania*. * **Treatment:** *M. marinum* is typically treated with Clarithromycin, Rifampicin, or Ethambutol for 3–6 months. * **Culture:** It grows on Lowenstein-Jensen (L-J) medium but requires a lower incubation temperature (30°C) to thrive.

Question 84: Boils can occur at all sites except which of the following?

A. Pinna
B. Skin
C. Scalp
D. Palm (Correct Answer)

Explanation: **Explanation:** The correct answer is **Palm**. **1. Underlying Medical Concept:** A **boil (furuncle)** is defined as a deep-seated infection of the **hair follicle**, most commonly caused by *Staphylococcus aureus*. By definition, a furuncle can only occur in areas where hair follicles are present. The **palms and soles** are characterized by **glabrous skin**, which is unique because it lacks hair follicles and sebaceous glands. Therefore, it is anatomically impossible for a boil to develop on the palm. **2. Analysis of Other Options:** * **Pinna:** The skin covering the cartilaginous part of the ear contains fine vellus hair follicles. Furuncles in the external auditory canal are common and extremely painful due to the tight adherence of the skin to the perichondrium. * **Skin:** This is a general term, but most body skin (except palms/soles) contains hair follicles and is a potential site for boils. * **Scalp:** The scalp has the highest density of terminal hair follicles in the body, making it a very common site for furunculosis. **3. NEET-PG High-Yield Clinical Pearls:** * **Definition Hierarchy:** * **Folliculitis:** Superficial infection of the hair follicle. * **Furuncle (Boil):** Deep-seated infection of a single hair follicle. * **Carbuncle:** A cluster of interconnected furuncles (multiple heads) involving the subcutaneous tissue, most common on the nape of the neck. * **Commonest Organism:** *Staphylococcus aureus* is the most frequent causative agent for all the above. * **Predisposing Factors:** Diabetes mellitus (always screen for DM in recurrent furunculosis), friction, and nasal carriage of Staph. * **Treatment:** Incision and drainage (I&D) is the primary treatment for fluctuant lesions; systemic antibiotics are used if cellulitis or systemic symptoms are present.

Question 85: What is a characteristic finding in borderline leprosy?

A. Inveed Saucer appearance (Correct Answer)
B. Erythema Nodosum Leprosum
C. Hypopigmented macules/plaques all over the body
D. Glove and Stocking Anesthesia

Explanation: **Explanation:** The correct answer is **A. Inverted Saucer appearance**. In leprosy, the clinical presentation depends on the host's cell-mediated immunity (CMI). **Borderline Leprosy (BB)** is the most unstable form, sitting exactly in the middle of the Ridley-Jopling scale. The characteristic lesion is a large, erythematous plaque with a punched-out, clear center and sloping outer edges, resembling an **"inverted saucer"** or "Swiss cheese" appearance. These lesions are often asymmetrical and have dry surfaces with variable sensory loss. **Analysis of Incorrect Options:** * **B. Erythema Nodosum Leprosum (ENL):** This is a Type 2 Lepra Reaction, typically seen in **Lepromatous (LL)** or **Borderline Lepromatous (BL)** leprosy due to immune complex deposition. It presents as tender, evanescent nodules. * **C. Hypopigmented macules/plaques all over the body:** While hypopigmented lesions occur in many types, "all over the body" (generalized, symmetrical distribution) is a hallmark of **Lepromatous Leprosy (LL)**. * **D. Glove and Stocking Anesthesia:** This is a classic feature of **Lepromatous Leprosy (LL)**, resulting from symmetrical, distal polyneuropathy. In Borderline types, nerve involvement is usually asymmetrical. **High-Yield Clinical Pearls for NEET-PG:** * **BB Leprosy:** Most unstable form; can upgrade toward Tuberculoid (BT) or downgrade toward Lepromatous (BL). * **Satellite lesions:** Small lesions near a larger plaque, characteristic of **BT (Borderline Tuberculoid)** leprosy. * **Leonine Facies:** Advanced **LL** leprosy due to diffuse infiltration of the face. * **Bacteriological Index (BI):** Usually 0 in TT, but strongly positive (4+ to 6+) in LL. BB usually has a BI of 3+ to 4+.

Question 86: Hansen's disease is another name for which of the following conditions?

A. Leprosy (Correct Answer)
B. Tuberculosis
C. Diabetes
D. Lichen planus

Explanation: **Explanation:** **Hansen’s disease** is the eponym for **Leprosy**, a chronic infectious disease caused by the acid-fast bacillus *Mycobacterium leprae*. It is named after the Norwegian physician Gerhard Armauer Hansen, who identified the bacterium in 1873. The disease primarily affects the skin, peripheral nerves, mucosa of the upper respiratory tract, and the eyes. **Analysis of Options:** * **Leprosy (Correct):** It is characterized by a spectrum of clinical presentations (Tuberculoid to Lepromatous) depending on the host's cell-mediated immunity. * **Tuberculosis:** Caused by *Mycobacterium tuberculosis*. While related to the leprosy bacillus, it is not referred to as Hansen’s disease. Cutaneous TB includes conditions like Lupus vulgaris and Scrofuloderma. * **Diabetes:** A metabolic disorder of glucose metabolism. While it can cause skin manifestations (e.g., Acanthosis nigricans, Necrobiosis lipoidica), it is unrelated to Hansen’s disease. * **Lichen planus:** An idiopathic inflammatory condition affecting the skin and mucous membranes, characterized by the "6 Ps" (Planar, Purple, Polygonal, Pruritic, Papules, and Plaques). **High-Yield Clinical Pearls for NEET-PG:** * **Cardinal Signs:** Hypopigmented/erythematous patches with loss of sensation, thickened peripheral nerves, and a positive skin smear for AFB. * **Nerve Involvement:** The **Ulnar nerve** is the most commonly involved nerve in the upper limb; the **Common Peroneal nerve** in the lower limb. * **Drug of Choice:** Dapsone, Rifampicin, and Clofazimine (MDT - Multidrug Therapy). * **World Leprosy Day:** Observed on the last Sunday of January (in India, Jan 30th).

Question 87: Which of the following organisms is responsible for vesicular eruptions and 'honey-coloured' crusts over the face?

A. TB bacilli
B. Staphylococcus aureus (Correct Answer)
C. Hansen's bacilli
D. Herpes zoster

Explanation: The clinical presentation described—**vesicular eruptions** progressing to **'honey-colored' crusts**—is the classic hallmark of **Impetigo Contagiosa** (Non-bullous impetigo). ### 1. Why Option B is Correct **Staphylococcus aureus** is currently the most common cause of non-bullous impetigo worldwide, followed by *Streptococcus pyogenes* (Group A Beta-hemolytic Strep). The pathogenesis involves the bacteria invading the superficial epidermis (subcorneal layer), leading to fragile vesicles that rupture easily. The dried exudate results in the characteristic "stuck-on" golden-yellow or honey-colored crusts, typically found on the face around the nose and mouth. ### 2. Why Other Options are Incorrect * **A. TB bacilli:** Cutaneous tuberculosis (e.g., Lupus vulgaris or Scrofuloderma) typically presents as chronic plaques, nodules, or cold abscesses, not acute vesicular eruptions with honey-colored crusts. * **C. Hansen's bacilli:** *Mycobacterium leprae* causes Leprosy, characterized by hypopigmented patches with loss of sensation or thickened nerves, rather than superficial crusting infections. * **D. Herpes zoster:** While it presents with vesicles, they are typically **grouped**, painful, and follow a **dermatomal distribution**. The crusts are usually hemorrhagic or serous, not classically "honey-colored." ### 3. NEET-PG High-Yield Pearls * **Bullous Impetigo:** Always caused by *S. aureus* (Phage group II). It is mediated by **Exfoliative Toxin A**, which targets **Desmoglein-1**. * **Ecthyma:** A deeper form of impetigo (extending into the dermis) that results in "punched-out" ulcers; often caused by *Streptococcus*. * **Complication:** While non-bullous impetigo can lead to **Post-Streptococcal Glomerulonephritis (PSGN)**, it does *not* lead to Rheumatic Fever. * **Treatment:** Topical **Mupirocin** is the drug of choice for localized lesions. For systemic involvement, use Cloxacillin or Amoxicillin-Clavulanate.

Question 88: Which of the following is a bacterial infection of the skin?

A. Pyoderma gangrenosum
B. Piedra
C. Impetigo contagiosa (Correct Answer)
D. Impetigo herpetiformis

Explanation: **Explanation:** **Impetigo contagiosa** is a highly contagious, superficial bacterial infection of the skin, primarily caused by **Staphylococcus aureus** and occasionally by **Streptococcus pyogenes** (Group A Beta-hemolytic Strep). It typically presents as thin-walled vesicles or pustules that rupture to form characteristic **"honey-colored" crusts**. It is most common in children and is a classic example of a primary pyoderma. **Analysis of Incorrect Options:** * **Pyoderma gangrenosum:** Despite the name, this is **not** an infectious condition. It is a sterile, inflammatory neutrophilic dermatosis often associated with systemic diseases like Inflammatory Bowel Disease (IBD) or Rheumatoid Arthritis. It presents as painful, rapidly expanding ulcers with undermined violaceous borders. * **Piedra:** This is a **fungal infection** of the hair shaft. White piedra is caused by *Trichosporon* species, while black piedra is caused by *Piedraia hortae*. * **Impetigo herpetiformis:** This is a misnomer. It is actually a rare, severe variant of **pustular psoriasis** occurring during pregnancy. It is not bacterial and is characterized by systemic symptoms like fever and hypocalcemia. **High-Yield Clinical Pearls for NEET-PG:** * **Bullous Impetigo:** Always caused by *S. aureus* producing exfoliative toxins (targeting Desmoglein 1). * **Ecthyma:** A deeper, "punched-out" ulcerative form of impetigo that heals with scarring. * **Post-Streptococcal Glomerulonephritis (PSGN):** Can follow impetigo caused by nephritogenic strains of Streptococcus, but Rheumatic Fever does **not** follow skin infections.

Question 89: What type of leprosy is characterized by an 'inveed saucer' appearance?

A. Borderline Tuberculoid (BT)
B. Lepidomatosis Leprosy (LL) (Correct Answer)
C. Borderline Borderline (BB)
D. Tuberculoid Tuberculoid (TT)

Explanation: ### Explanation The **'inverted saucer'** appearance is a classic morphological description of the skin lesions found in **Lepromatous Leprosy (LL)**. **1. Why Lepromatous Leprosy (LL) is correct:** In LL, the cell-mediated immunity (CMI) is severely depressed or absent. This leads to an uncontrolled proliferation of *Mycobacterium leprae*. The skin lesions are typically numerous, small, bilateral, and symmetrical. As the disease progresses, these macules evolve into papules and nodules (lepromas). The "inverted saucer" appearance refers to these **nodular lesions or plaques** that are characteristically elevated in the center and slope down towards the periphery, resembling a saucer turned upside down. **2. Analysis of Incorrect Options:** * **Borderline Tuberculoid (BT):** Characterized by fewer, asymmetrical lesions with well-defined borders. A high-yield feature here is "satellite lesions" around a larger plaque. * **Borderline Borderline (BB):** This is the most unstable form. The characteristic lesion is the **"punched-out"** or **"Swiss cheese"** appearance (annular plaques with a clear, well-defined inner edge and a hazy outer edge). * **Tuberculoid Tuberculoid (TT):** Features a single or very few, large, anesthetic, hairless, hypopigmented plaques with sharply defined, raised borders. **3. NEET-PG Clinical Pearls:** * **Leonine Facies:** Advanced LL with diffuse infiltration of the face, loss of eyebrows (madarosis), and thickening of the nose/ears. * **Grenz Zone:** A clear sub-epidermal zone seen on histology in LL, where the dermis is separated from the epidermis by a band of normal collagen. * **Bacteriological Index (BI):** Highest in LL (4+ to 6+), as the body cannot contain the bacilli. * **Globi:** Large clusters of bacilli found within **Virchow cells** (foamy macrophages) in LL.

Question 90: Lepra cells are seen in abundance in which type of leprosy?

A. Tuberculoid leprosy
B. Lepromatous leprosy (Correct Answer)
C. Histoid leprosy
D. Intermediate leprosy

Explanation: **Explanation:** The presence of **Lepra cells** (also known as **Virchow cells**) is a hallmark of **Lepromatous Leprosy (LL)**. These are large, foamy macrophages that have ingested a high number of *Mycobacterium leprae*. In LL, the patient exhibits a deficient Cell-Mediated Immunity (CMI), allowing the bacilli to multiply unchecked. The macrophages become "stuffed" with bacilli, which are often arranged in parallel clusters called **globi**. **Analysis of Options:** * **Lepromatous Leprosy (Correct):** Due to the Th2-dominant cytokine profile and poor CMI, there is a high bacillary load (multibacillary). Macrophages fail to kill the bacteria, leading to the formation of foamy Lepra cells. * **Tuberculoid Leprosy (Incorrect):** This pole is characterized by strong CMI (Th1 response). Bacilli are very rare (paucibacillary), and the histology shows well-formed granulomas with epithelioid cells and Langhans giant cells, rather than foamy Lepra cells. * **Histoid Leprosy (Incorrect):** A variant of LL characterized by spindle-shaped histiocytes arranged in a storiform pattern. While it has a high bacillary load, the characteristic cell is the spindle cell, not the classic foamy Lepra cell. * **Indeterminate Leprosy (Incorrect):** This is an early stage where the immune response hasn't polarized. Histology usually shows only non-specific perineural inflammation. **NEET-PG High-Yield Pearls:** * **Grenz Zone:** A clear subepidermal zone of uninvolved dermis seen in Lepromatous and Histoid leprosy (absent in Tuberculoid). * **Fite-Faraco Stain:** A modified acid-fast stain used to visualize *M. leprae* in tissue sections. * **Globi:** Masses of acid-fast bacilli held together by a mucoid material called **glis**. * **Ridley-Jopling Classification:** The standard 5-group classification based on the immunological spectrum of the disease.

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