Clin Med: Infectious Disease Presentation and Pathophysiology

Tinea Capitis

A superficial fungal infection of the scalp and hair shafts, most commonly caused by dermatophytes of the genus Trichophyton or Microsporum. It presents primarily in children as patchy alopecia, scaling of the scalp, mild pruritus, broken hairs resembling "black dots," and classic occipital or posterior cervical lymphadenopathy; it can also manifest as diffuse scaling resembling severe dandruff, kerion, or favus.

Trichophyton

A dermatophyte most common in the US

Microsporum

A dermatophyte most commonly from an animal exposure

Tinea Corporis ("Ringworm")

A superficial dermatophyte infection of the glabrous (non-hair-bearing) skin, most commonly caused by Trichophyton rubrum. It classically presents on exposed areas of the body as annular erythematous plaques with a raised, scaly advancing border and central clearing, slowly enlarging centrifugally and causing mild to moderate pruritus.

Trichophyton rubrum

Causes tinea corporis and tinea unguium

Tinea Cruris ("Jock Itch")

A superficial dermatophyte infection of the groin area, heavily associated with risk factors such as male sex, obesity, excessive sweating, occlusive clothing, and concurrent tinea pedis. It is confined to the groin and gluteal cleft while characteristically sparing the scrotum, presenting as well-demarcated erythematous plaques with an elevated, scaly advancing border, central clearing, and occasional association with follicular papules or pustules.

Tinea Pedis ("Athlete's Foot")

A dermatophyte infection of the feet triggered by risk factors like occlusive footwear, sweating, public showers/locker rooms, and diabetes. It manifests either as the interdigital type, characterized by scaling, maceration, pruritus, and painful fissures between the toes (especially the 4th–5th web spaces), or as the moccasin type, presenting with diffuse plantar scaling, hyperkeratosis, and thickening and fissuring of the sole and heel.

Tinea Manuum

A dermatophyte infection of the hands that often occurs concurrently with tinea pedis in a classic "two feet, one hand" syndrome due to autoinoculation. It presents with scaling, hyperkeratosis, pruritus, burning, or stinging on the hands.

Tinea Versicolor

A superficial skin infection caused by Malassezia, which is a normal skin yeast rather than a dermatophyte. It presents on the upper back, chest, and shoulders as well-demarcated hypo-, hyper-, or pink-colored macules that may coalesce into larger patches with a fine, subtle scale that becomes apparent when gently scraped (the "scratch sign" or evoked scale), and pigment changes that become more noticeable after sun exposure because the affected skin fails to tan normally.

Malassezia

normal skin yeast that causes tinea versicolor

Tinea Unguium (Onychomycosis)

A dermatophyte infection of the nails, most commonly toenails, typically caused by Trichophyton rubrum and often spreading from concurrent tinea pedis. It presents as a thickened nail with subungual hyperkeratosis, yellow or white discoloration, a brittle, crumbly texture, and distal onycholysis beginning at the distal nail edge.

Cutaneous Candidiasis

A superficial fungal infection of cutaneous surfaces involving body folds (intertriginous areas) and triggered by risk factors such as diabetes, pregnancy, obesity, warm moist environments, and immunosuppression. It presents with intense pruritus or burning, moist macerated skin, and bright red erythematous plaques flanked by characteristic pinpoint satellite papules or pustules ("satellite lesions").

Angular Cheilitis

A localized candida-associated inflammatory process at the labial commissures, often triggered by denture use or excessive moisture. It presents as erythema, maceration, and painful fissuring localized at the corners of the mouth.

Oral Thrush (Oral Candidiasis)

A superficial fungal infection of the oral mucous membranes caused by Candida overgrowth, commonly associated with denture use or immunosuppressive states. It presents as thick, white, curdlike mucosal plaques that can be easily wiped away to reveal a raw, underlying erythematous base.

Sporotrichosis ("Rose Gardener's Disease")

A deep fungal infection caused by organisms of the Sporothrix schenckii complex, typically introduced via traumatic inoculation from rose thorns, moss, hay, or decaying vegetation into the skin. It presents initially as a painless papule or nodule at the inoculation site that slowly ulcerates over time, followed by the sequential development of new nodules spreading proximally along draining lymphatic channels ("sporotrichoid spread").

Sporothrix schenckii complex

Fungus that causes Sporotrichosis

Blastomycosis

A deep fungal infection caused by inhalation of spores from moist soil and decaying wood, endemic to the Ohio and Mississippi River Valleys, Great Lakes region, and southeastern US, primarily affecting individuals in outdoor occupations. It begins as a primary pulmonary infection (causing cough, fever, dyspnea, and chest pain) but can disseminate to the skin, presenting as raised verrucous (wart-like) or ulcerative skin lesions.

Histoplasmosis

A deep fungal infection caused by inhaling spores of Histoplasma capsulatum found in soil contaminated with bird or bat droppings. It is often asymptomatic or a mild flu-like pulmonary illness but can progress to disseminated disease (especially in immunocompromised patients), presenting with fever, weight loss, hepatosplenomegaly, and disseminated papular or nodular skin lesions.

Coccidioidomycosis ("Valley Fever")

A deep fungal infection acquired by inhaling airborne arthroconidia from desert soil, endemic to the southwestern US, northern Mexico, and South America. It presents as a common cause of community-acquired pneumonia in endemic regions with symptoms of fever, cough, chest pain, and fatigue, frequently accompanied by arthralgias and tender, red nodules on the shins (erythema nodosum).

arthroconidia

fungal spores that cause Coccidioidomycosis

Cryptococcosis

A systemic “deep” fungal infection caused by Cryptococcus neoformans, acquired via inhalation of encapsulated yeast found in soil contaminated with pigeon droppings. It primarily manifests as pulmonary disease but can disseminate (especially in patients with HIV/AIDS) to cause severe fungal meningitis or characteristic umbilicated skin papules and nodules resembling molluscum contagiosum.

Mycetoma

A chronic, progressive, destructive, deep fungal infection (most commonly Madurella mycetomatis) acquired through traumatic subcutaneous soil inoculation from walking barefoot, splinters, or thorns. It begins in the subcutaneous tissue of the foot and can extend into muscle and bone, presenting as a classic clinical triad of a painless subcutaneous mass, multiple draining sinus tracts, and visible granules ("grains") in the purulent discharge.

Cellulitis

An acute, diffuse bacterial infection of the deep dermis and subcutaneous tissue, most commonly caused by beta-hemolytic streptococci (especially Group A Streptococcus) or Staphylococcus aureus. It classically begins as a small area of skin barrier disruption (such as from interdigital tinea pedis) that rapidly expands over hours to days, presenting with ill-defined borders, diffuse swelling, erythema, warmth, pain/tenderness, and occasional systemic symptoms like fever and chills.

Erysipelas

A superficial bacterial skin infection involving the upper dermis and superficial lymphatics, typically caused by Group A Streptococcus (Streptococcus pyogenes). It presents with a rapid onset of a painful, bright-red, warm erythematous plaque featuring sharply demarcated, elevated borders, warm, tense, glistening skin, and occasional vesicle or bullae formation, most commonly involving the face or lower extremities and accompanied by sudden fever and chills.

Impetigo

A highly contagious, superficial bacterial infection of the epidermal layer of the skin, most commonly caused by Staphylococcus aureus or Group A Streptococcus. It primarily affects school-age children, presenting on the face (especially around the nose and mouth) as pruritic vesicles or pustules that rupture easily to leave behind honey-colored crusts.

Ecthyma

An ulcerative form of impetigo that extends deeply into the dermal layer of the skin, usually caused by Group A Streptococcus or Staphylococcus aureus. It is more common in patients with poor hygiene, chronic edema, or diabetes, presenting as vesicles or pustules on the lower extremities that progress into painful, punched-out ulcers covered by a thick, adherent gray-yellow crust, which characteristically heal with scarring.

Staphylococcal Scalded Skin Syndrome

A toxin-mediated dermatosis caused by hematogenous spread of exfoliative toxins produced by certain strains of Staphylococcus aureus. It primarily affects children under 5 years old, presenting initially with fever, irritability, and a URI-like prodrome, rapidly progressing to diffuse, tender erythema starting on the face and flexures, followed by flaccid bullae that rupture easily to cause widespread desquamation (resembling a severe scald burn) and a positive Nikolsky sign.

Folliculitis

A superficial bacterial or chemical inflammation of one or more hair follicles, most commonly caused by Staphylococcus aureus, Pseudomonas (from contaminated hot tubs), Gram-negative bacteria, or friction/oils. It presents as localized, small pustules centered on hair follicles accompanied by burning, tenderness, or itching.

Furuncle ("Boil")

A deep bacterial infection of a hair follicle extending into the surrounding dermis and subcutaneous tissue, typically caused by Staphylococcus aureus (including MRSA). It presents as a painful, erythematous, fluctuant nodule that gradually enlarges over several days before spontaneously draining purulent and necrotic material.

Carbuncle

A deep-seated, extensive bacterial abscess formed by the coalescence of multiple adjacent furuncles, usually caused by Staphylococcus aureus. It presents as a large, painful, fluctuant subcutaneous mass with multiple draining sinus tracts, often accompanied by surrounding cellulitis, systemic fever, and chills.

Hidradenitis Suppurativa (Acne Inversa)

A chronic, recurrent inflammatory skin condition caused by follicular occlusion, follicular rupture, and subsequent chronic inflammation of intertriginous areas (such as axillae, groin, inframammary folds, and buttocks). It typically presents after puberty as recurrent, deep, painful nodules (the hallmark initial lesion), sterile abscesses, draining sinus tracts, and progressive hypertrophic scarring.

Pediculosis capitis, corporis, or pubis

An infestation of the scalp, trunk, or pubic areas by ectoparasitic lice transmitted via direct contact. It presents with intense pruritus secondary to a hypersensitivity reaction to louse saliva, excoriations, and visible live lice or nits firmly cemented to hair shafts near the scalp/pubis or concentrated within the seams of clothing.

Scabies

An infestation of the stratum corneum by the microscopic Sarcoptes scabiei mite, transmitted primarily via prolonged skin-to-skin contact. It presents with intense, widespread pruritus that characteristically worsens at night, excoriations, pruritic papules, vesicles, and pathognomonic thin, wavy, gray-white burrows distributed classically in finger webs, wrists, axillae, umbilicus, and genitals, with characteristic nodular lesions on the scrotum or penis

Kerion

boggy, edematous painful plaque

favus

yellow cup-shaped crusts that may result in permanent scarring alopecia

Griseofulvin

oral antifungal perferred for microsporum

terbinafine

oral antifungal for trichophyton

purulent cellulitis/abscess

MRSA

Nonpurulent cellulitis

Beta-hemolytic streptococci (esp Group A)

Freshwater injury

Aeromonas hydrophila

saltwater injury or oyster exposure

Vibrio vulnificus

Dog or cat bite

Pasteurella multocida

Fish tank or aquarium exposure

Mycobacterium marinum

Hot tub exposure

pseudomonas aeruginosa

Diabetic foot infection

Polymicrobial

Nikolsky sign

a medical skin test where gentle rubbing or sideways pressure on the skin causes the top layer to peel or slide off