Infectious skin diseases US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Infectious skin diseases. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Infectious skin diseases US Medical PG Question 1: A 14-month-old boy is brought to the clinic for evaluation of a rash. The rash started on the face and spread to the trunk. He also had a fever and cough for the past 2 days. His mother says that they recently immigrated from Asia and cannot provide vaccination records. The physical examination reveals a maculopapular rash on the face, trunk, and proximal limbs with no lymphadenopathy. Blue-white spots are noted on the oral mucosa and there is bilateral mild conjunctival injection. The causative agent of this condition belongs to which of the following virus families?
- A. ssRNA naked viruses
- B. ssRNA enveloped viruses (Correct Answer)
- C. dsRNA naked viruses
- D. dsRNA enveloped viruses
- E. ssDNA enveloped viruses
Infectious skin diseases Explanation: ***ssRNA enveloped viruses***
- The clinical presentation with **maculopapular rash** spreading from face to trunk, **fever**, **cough**, **conjunctivitis**, and especially **Koplik's spots** (blue-white spots on oral mucosa) is pathognomonic for **measles** (rubeola).
- Measles virus is a **single-stranded RNA (ssRNA) enveloped virus** belonging to the **Paramyxoviridae family**.
- The envelope contains hemagglutinin and fusion proteins that facilitate viral entry.
*ssRNA naked viruses*
- Includes viruses like picornaviruses (rhinovirus, enterovirus) and caliciviruses (norovirus).
- These cause respiratory infections or gastroenteritis, not the classic measles presentation with Koplik's spots.
*dsRNA naked viruses*
- Example: **Rotavirus** (Reoviridae family), which causes gastroenteritis in children.
- Does not present with maculopapular rash or Koplik's spots.
*dsRNA enveloped viruses*
- Extremely rare in human pathology; no common human disease fits this category.
- Not relevant to measles-like presentations.
*ssDNA enveloped viruses*
- Very rare category; most DNA viruses are dsDNA.
- No human disease with maculopapular rash and Koplik's spots is caused by ssDNA enveloped viruses.
Infectious skin diseases US Medical PG Question 2: Physical exam of a 15-year-old female reveals impetigo around her mouth. A sample of the pus is taken and cultured. Growth reveals gram-positive cocci in chains that are bacitracin sensitive. Which of the following symptoms would be concerning for a serious sequela of this skin infection?
- A. Myocarditis
- B. Joint swelling
- C. Fever
- D. Chorea
- E. Hematuria (Correct Answer)
Infectious skin diseases Explanation: ***Hematuria***
- The description of **impetigo** around the mouth caused by **gram-positive cocci in chains** that are **bacitracin-sensitive** points to *Streptococcus pyogenes* (Group A Streptococcus).
- A serious sequela of streptococcal skin infections (impetigo) is **post-streptococcal glomerulonephritis (PSGN)**, which presents with **hematuria**, proteinuria, edema, and hypertension.
*Myocarditis*
- **Myocarditis** can be a complication of **acute rheumatic fever (ARF)**, which is a sequela of **streptococcal pharyngitis**, not typically streptococcal skin infections (impetigo).
- While both rheumatic fever and PSGN are caused by *Streptococcus pyogenes*, the specific **M-types** associated with skin infections differ from those causing pharyngitis and ARF.
*Joint swelling*
- **Arthritis** (joint swelling) is a major criterion for **acute rheumatic fever**, which follows **streptococcal pharyngitis**, not impetigo.
- Although PSGN can cause arthralgia, significant arthritis is not a primary or alarming symptom of PSGN.
*Fever*
- **Fever** is a general symptom that can accompany any infection, including impetigo itself or many other conditions.
- While fever can be present in PSGN, it is not a specific or unique indicator of this particular serious sequela.
*Chorea*
- **Sydenham's chorea** is a neurological manifestation of **acute rheumatic fever**, which develops after **streptococcal pharyngitis**, not impetigo.
- It presents as involuntary, purposeless movements and is not associated with PSGN following impetigo.
Infectious skin diseases US Medical PG Question 3: A 37-year-old woman with a history of systemic lupus erythematosus, on prednisone and methotrexate, presents to the dermatology clinic with three weeks of a diffuse, itchy rash. Physical exam is remarkable for small red papules in her bilateral axillae and groin and thin reddish-brown lines in her interdigital spaces. The following skin biopsy is obtained. Which of the following is the most appropriate treatment?
- A. Capsaicin cream
- B. Ketoconazole cream
- C. Permethrin cream (Correct Answer)
- D. Hydrocortisone cream
- E. Nystatin cream
Infectious skin diseases Explanation: ***Permethrin cream***
- The patient's presentation with **diffuse itchy rash**, small red papules in the axillae and groin, and **reddish-brown lines in interdigital spaces (burrows)** is classic for **scabies**.
- **Permethrin 5% cream** is the **first-line treatment** for scabies due to its efficacy as a **scabicidal agent** by disrupting the parasite's nervous system.
- The patient's **immunocompromised status** (on prednisone and methotrexate) increases risk for **crusted (Norwegian) scabies**, but permethrin remains the primary topical treatment; severe cases may require addition of oral ivermectin.
*Capsaicin cream*
- Capsaicin cream is used for **neuropathic pain** and often causes a burning sensation, making it unsuitable for a pruritic rash caused by mites.
- It does not have any **antiparasitic properties** and would not treat the underlying cause of scabies.
*Ketoconazole cream*
- Ketoconazole is an **antifungal agent** used to treat conditions like candidiasis or tinea infections.
- The clinical presentation is not suggestive of a fungal infection, and ketoconazole would be ineffective against scabies mites.
*Hydrocortisone cream*
- Hydrocortisone is a **topical corticosteroid** used to reduce inflammation and itching associated with various dermatoses.
- While it may temporarily relieve itching, it would not eradicate the **scabies mites** and their eggs, leading to recurrence.
- Using corticosteroids alone in an **already immunocompromised patient** could worsen the infestation.
*Nystatin cream*
- Nystatin is another **antifungal medication** primarily used for cutaneous candidiasis.
- It has no activity against parasitic infestations such as scabies and would therefore be an inappropriate treatment.
Infectious skin diseases US Medical PG Question 4: A 3-year-old boy is brought to the physician because of a 3-day history of a pruritic skin rash on his chest. His mother says that he has no history of dermatological problems. He was born at term and has been healthy except for recurrent episodes of otitis media. His immunizations are up-to-date. He appears pale. His temperature is 37°C (98.6°F), pulse is 110/min, respirations are 26/min, and blood pressure is 102/62 mm Hg. Examination shows vesicles and flaccid bullae with thin brown crusts on the chest. Lateral traction of the surrounding skin leads to sloughing. Examination of the oral mucosa shows no abnormalities. Complete blood count is within the reference range. Which of the following is the most likely diagnosis?
- A. Bullous pemphigoid
- B. Dermatitis herpetiformis
- C. Bullous impetigo (Correct Answer)
- D. Pemphigus vulgaris
- E. Stevens-Johnson syndrome
Infectious skin diseases Explanation: ***Bullous impetigo***
- The presence of **flaccid bullae with thin brown crusts** and the positive **Nikolsky's sign** (sloughing with lateral traction), in the absence of mucosal involvement, are classic signs of bullous impetigo, a **Staphylococcus aureus** infection.
- This condition is common in children and can present with localized lesions, as seen on the chest.
*Bullous pemphigoid*
- Typically presents with **tense bullae** in older adults, often with **urticarial plaques**, unlike the flaccid bullae and crusts seen here.
- **Nikolsky's sign is negative** in bullous pemphigoid, which helps distinguish it from bullous impetigo and pemphigus conditions.
*Dermatitis herpetiformis*
- Characterized by intensely **pruritic papules and vesicles** found symmetrically on extensor surfaces, often associated with **celiac disease**.
- The lesions are usually small and grouped, not flaccid bullae with positive Nikolsky's sign.
*Pemphigus vulgaris*
- Presents with **flaccid bullae** and a positive Nikolsky's sign, but characteristically also involves the **oral mucosa**, which is normal in this patient.
- It usually affects older individuals and can be more widespread than the localized rash described.
*Stevens-Johnson syndrome*
- A severe mucocutaneous reaction typically characterized by **widespread epidermal necrosis**, **target lesions**, and often involves **mucous membranes** (oral, ocular, genital) extensively.
- This patient's localized rash without mucosal involvement, target lesions, or systemic toxicity does not fit the criteria for SJS.
Infectious skin diseases US Medical PG Question 5: A 25-year-old man presents with multiple brownish patches on his trunk that appeared suddenly after exercising in hot weather. The lesions don't itch or scale. Wood's lamp examination shows yellow-green fluorescence. Which of the following is the most appropriate treatment?
- A. Topical selenium sulfide (Correct Answer)
- B. Oral antibiotics
- C. Topical steroids
- D. Oral antifungal
Infectious skin diseases Explanation: ***Topical selenium sulfide***
- This presentation is highly suggestive of **tinea versicolor** (pityriasis versicolor), caused by the yeast *Malassezia globosa*.
- Topical selenium sulfide is a well-established and effective **antifungal agent** for treating tinea versicolor, often used as a shampoo or lotion.
- It is the **first-line treatment** for localized disease.
*Oral antibiotics*
- **Tinea versicolor** is a fungal infection, not bacterial, rendering antibiotics ineffective.
- Antibiotics are indicated for **bacterial infections** and would not address the underlying etiology here.
*Topical steroids*
- Topical steroids are **anti-inflammatory agents** and would not treat the fungal overgrowth causing tinea versicolor.
- They could potentially worsen fungal infections by **suppressing local immunity**.
*Oral antifungal*
- While oral antifungals (such as fluconazole or itraconazole) can be used for widespread or recurrent tinea versicolor, **topical therapy** is generally preferred as first-line treatment for localized disease.
- The initial presentation doesn't suggest the need for systemic treatment, making a **topical agent more appropriate** as the first choice due to fewer side effects and adequate efficacy.
Infectious skin diseases US Medical PG Question 6: A 32-year-old man with a past medical history significant for HIV and a social history of multiple sexual partners presents with new skin findings. His past surgical and family histories are noncontributory. The patient's blood pressure is 129/75 mm Hg, the pulse is 66/min, the respiratory rate is 16/min, and the temperature is 37.5°C (99.6°F). Physical examination reveals numerous painless skin-colored, flattened and papilliform lesions along the penile shaft and around the anus on physical exam. The application of 5% acetic acid solution causes the lesions to turn white. What is the etiology of these lesions?
- A. HSV (type 2)
- B. HPV (types 6 & 11) (Correct Answer)
- C. Neisseria gonorrhoeae
- D. HPV (types 16 & 18)
- E. Molluscum contagiosum
Infectious skin diseases Explanation: ***HPV (types 6 & 11)***
- These types of **Human Papillomavirus** are responsible for the vast majority of **genital warts (condyloma acuminata)**, which typically present as **painless, skin-colored, flattened, and papilliform lesions**.
- The lesions turning white upon application of **5% acetic acid solution (acetowhitening)** is a characteristic finding that helps visualize and identify HPV-related lesions.
*HSV (type 2)*
- **Herpes Simplex Virus type 2** causes **genital herpes**, which presents as painful, vesicular lesions that typically rupture to form ulcers.
- The lesions described in the question are painless and papilliform, not vesicular or ulcerated.
*Neisseria gonorrhoeae*
- **Neisseria gonorrhoeae** causes **gonorrhea**, a bacterial infection that typically presents as urethritis with purulent discharge in men, or can be asymptomatic.
- It does not cause wart-like lesions on the skin.
*HPV (types 16 & 18)*
- While these are high-risk types of **Human Papillomavirus**, they are primarily associated with **cervical, anal, and other anogenital cancers**, rather than benign genital warts.
- The lesions described are characteristic of condyloma acuminata, which are typically caused by low-risk HPV types.
*Molluscum contagiosum*
- **Molluscum contagiosum** manifests as **umbilicated papules**, meaning they have a central indention, which is distinct from the papilliform lesions described.
- These lesions are typically small, flesh-colored to pearly, and dome-shaped.
Infectious skin diseases US Medical PG Question 7: A 17-year-old boy comes to the physician because of a nonpruritic rash on his chest for 1 week. He returned from a trip to Puerto Rico 10 days ago. He started using a new laundry detergent after returning. He has type 1 diabetes mellitus controlled with insulin. His mother has Hashimoto thyroiditis, and his sister has severe facial acne. Examination of the skin shows multiple, nontender, round, hypopigmented macules on the chest and trunk. There is fine scaling when the lesions are scraped with a spatula. Which of the following is the most likely underlying mechanism of this patient's symptoms?
- A. Increased sebum production
- B. Infection with Trichophyton rubrum
- C. Increased growth of Malassezia globosa (Correct Answer)
- D. Antigen uptake by Langerhans cells
- E. Autoimmune destruction of melanocytes
Infectious skin diseases Explanation: ***Increased growth of Malassezia globosa***
- The clinical presentation of **hypopigmented macules with fine scaling** on the chest and trunk, especially in a young male who recently traveled to a tropical climate (Puerto Rico), is highly characteristic of **tinea versicolor**.
- Tinea versicolor is caused by an overgrowth of **Malassezia species** (primarily *M. globosa*), a commensal yeast that thrives in warm, humid conditions and is often associated with oily skin, leading to dyspigmentation and scaling due to interference with melanin production.
*Increased sebum production*
- While increased sebum production can contribute to a favorable environment for fungal growth, it is the underlying mechanism for conditions like **acne vulgaris** and **seborrheic dermatitis**, which present differently (e.g., comedones, inflammatory papules, erythema, greasy scales).
- This patient's rash is specifically described as hypopigmented macules with fine scaling, not inflammatory papules or pustules typical of acne, nor the greasy scales of seborrheic dermatitis.
*Infection with Trichophyton rubrum*
- **Trichophyton rubrum** is a dermatophyte commonly responsible for **tinea corporis** (ringworm of the body), **tinea pedis** (athlete's foot), and **tinea cruris** (jock itch), which typically present as **erythematous, annular, itchy lesions with raised borders**.
- The rash described here (hypopigmented, nonpruritic macules) does not fit the typical presentation of dermatophyte infections caused by *Trichophyton rubrum*.
*Antigen uptake by Langerhans cells*
- **Antigen uptake by Langerhans cells** is a crucial step in the pathogenesis of **allergic contact dermatitis**, where the skin reacts to an external allergen after sensitization, mediated by a **Type IV hypersensitivity reaction**.
- While the patient used a new laundry detergent, the rash's appearance (**hypopigmented macules with fine scaling**) is incongruent with contact dermatitis, which usually presents as an **erythematous, pruritic, vesicular, or eczematous rash** in the contact area.
*Autoimmune destruction of melanocytes*
- **Autoimmune destruction of melanocytes** is the underlying mechanism for **vitiligo**, an autoimmune condition characterized by **well-demarcated, completely depigmented patches** of skin.
- The lesions in this patient are described as **hypopigmented** (reduced pigmentation), not completely depigmented, and show **fine scaling**, which is not a feature of vitiligo.
Infectious skin diseases US Medical PG Question 8: A 59-year-old man presents to the health clinic for evaluation of severe itching for the past week. The itching is worse at night while lying in bed. The patient has a past medical history of hyperlipidemia, atrial fibrillation, and colon cancer. The patient takes rivaroxaban, simvastatin, and aspirin. The patient has a surgical history of colon resection, appendectomy, and tonsillectomy. He drinks a 6-pack of beer almost every night of the week. He smokes 2 packs of cigarettes daily and has been living at a homeless shelter for the past 6 months. Examination of the skin shows small crusted sores and superficial, wavy gray lines along the wrists and interdigital spaces of both hands as seen in the image. Small vesicles are also present along with excoriations. Which of the following is the most appropriate treatment option for this patient?
- A. Ivermectin
- B. Dicloxacillin
- C. Penicillin G
- D. Acyclovir
- E. Permethrin (Correct Answer)
Infectious skin diseases Explanation: ***Permethrin***
- The patient's symptoms of **severe itching**, especially **worse at night**, and the presence of **crusted sores, wavy gray lines (burrows)** on the wrists and interdigital spaces are highly characteristic of **scabies**.
- **Permethrin cream** (5%) is a first-line topical treatment for scabies due to its high efficacy against *Sarcoptes scabiei* mites.
*Ivermectin*
- While **oral ivermectin** is effective for scabies, particularly **crusted scabies** or when topical treatment fails, it is typically considered a second-line agent.
- Given the classic presentation, a topical approach like permethrin is usually tried first, especially if the patient is not severely immunocompromised or has extensive crusted lesions.
*Dicloxacillin*
- **Dicloxacillin** is an **antibiotic** used to treat **bacterial infections**, commonly staphylococcal and streptococcal skin infections.
- While the patient has excoriations and crusted sores, suggesting potential secondary bacterial infection, the primary infestation diagnosis is scabies, which requires an **acaricide**, not an antibiotic.
*Penicillin G*
- **Penicillin G** is an **antibiotic** primarily used for treating severe bacterial infections, particularly those caused by susceptible streptococci, meningococci, and specific spirochetes.
- It is **ineffective against parasitic infestations** like scabies, which is caused by mites.
*Acyclovir*
- **Acyclovir** is an **antiviral medication** used to treat herpes simplex virus (HSV) infections (e.g., cold sores, genital herpes) and varicella-zoster virus (VZV) infections (e.g., shingles, chickenpox).
- The patient's clinical presentation is inconsistent with a viral infection; therefore, **acyclovir would not be an appropriate treatment** for scabies.
Infectious skin diseases US Medical PG Question 9: A 10-month-old girl is brought to the clinic by her mother with skin lesions on her chest. The mother says that she noticed the lesions 24 hours ago and that they have not improved. The patient has no significant past medical history. She was born at term by spontaneous transvaginal delivery with no complications, is in the 90th percentile on her growth curve, and has met all developmental milestones. Upon physical examination, several skin-colored umbilicated papules are visible. Which of the following is the most appropriate treatment of this patient's likely diagnosis?
- A. Cryotherapy or podophyllotoxin (0.15% topically)
- B. Observation/watchful waiting (Correct Answer)
- C. Acyclovir
- D. Wide-spectrum antibiotics
- E. Topical antifungal therapy
Infectious skin diseases Explanation: ***Observation/watchful waiting***
- The patient's presentation with **skin-colored umbilicated papules** is classic for **molluscum contagiosum**, a benign self-limited viral infection caused by a **poxvirus**.
- In **immunocompetent children**, **observation is the first-line management** as most cases resolve spontaneously within **6-18 months** without intervention.
- The lesions have only been present for **24 hours**, and the patient is a healthy **10-month-old infant** with no complications, making watchful waiting the most appropriate approach.
- Active treatment is typically reserved for **immunocompromised patients**, **extensive or persistent lesions**, or cases with **significant psychosocial impact** or cosmetic concerns.
*Cryotherapy or podophyllotoxin (0.15% topically)*
- While these can be used for molluscum contagiosum, they are **not first-line** in healthy young children.
- **Podophyllotoxin** is generally **contraindicated in children under 2 years** due to safety concerns and potential toxicity.
- **Cryotherapy** is painful and can cause scarring, making it inappropriate as initial management in a **10-month-old infant** with recent-onset lesions.
- These treatments may be considered for **persistent cases** after a period of observation or in specific circumstances.
*Acyclovir*
- **Acyclovir** is an antiviral medication effective against **herpes simplex virus (HSV)** and **varicella-zoster virus (VZV)**.
- It has **no activity against poxviruses** and is not indicated for **molluscum contagiosum**.
*Wide-spectrum antibiotics*
- **Antibiotics** are effective against **bacterial infections** only and have no role in treating **viral skin infections** like molluscum contagiosum.
- Inappropriate antibiotic use contributes to **antimicrobial resistance**.
*Topical antifungal therapy*
- **Antifungal medications** treat **fungal infections** such as tinea (ringworm) or candidiasis.
- They are not effective against **molluscum contagiosum**, which is a **viral infection**.
Infectious skin diseases US Medical PG Question 10: A 24-year-old man presents to the emergency department with sudden onset of fever for the past few hours as well as pain and swelling in his right knee and left ankle. He denies any recent history of trauma or injury. The patient is otherwise a healthy, active young man. He recently recovered from a case of gastroenteritis which caused significant abdominal pain and bloody stool 4 weeks ago. He believes the infection was related to eating undercooked chicken while camping. His blood pressure is 124/76 mm Hg, his heart rate is 76/min, and his temperature is 36.9 ℃ (98.4 ℉). Physical examination reveals tenderness to palpation of his right knee and left ankle as well as erythematous conjunctiva. Which of the following features would be least likely to develop in patients with this condition?
- A. Circinate balanitis
- B. Genital ulcers
- C. DIP joint swelling (Correct Answer)
- D. Urethritis
- E. Skin rash
Infectious skin diseases Explanation: ***DIP joint swelling***
- **Reactive arthritis** typically involves the **large joints** of the lower extremities in an asymmetric pattern, such as the knees and ankles, but spares the **distal interphalangeal (DIP) joints**.
- The patient's history of recent gastroenteritis, subsequent arthritis, and conjunctivitis are classic features of reactive arthritis (formerly Reiter's syndrome), which is a form of **seronegative spondyloarthropathy**.
*Circinate balanitis*
- **Circinate balanitis** is a painless, shallow ulceration of the glans penis that is a characteristic **mucocutaneous manifestation** of reactive arthritis.
- This condition occurs in a significant number of male patients with **HLA-B27 positive** reactive arthritis.
*Genital ulcers*
- **Genital ulcers** are possible cutaneous manifestations of reactive arthritis.
- These can present along with other skin findings such as **keratoderma blennorrhagicum** (pustular psoriasis-like lesions) and circinate balanitis.
*Urethritis*
- **Urethritis** is a common component of the classic triad of symptoms in reactive arthritis ("can't pee, can't see, can't climb a tree").
- It manifests as **dysuria, urinary frequency**, or penile discharge, often following a gastrointestinal or genitourinary infection.
*Skin rash*
- A skin rash, particularly **keratoderma blennorrhagicum**, which resembles pustular psoriasis, is a known *cutaneous manifestation* of reactive arthritis.
- Lesions typically appear on the **palms and soles**, but can also affect the trunk and scalp.
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