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Summary Table

Category	Details
Risk Factors	Adolescents and young adults (peak 14 to 19 years), family history of severe acne, androgen excess states (PCOS, congenital adrenal hyperplasia), use of comedogenic cosmetics, lithium, corticosteroids, anabolic steroids, phenytoin
Etiology	Multifactorial: androgen-driven sebum overproduction, follicular hyperkeratinization, Cutibacterium acnes colonization, and subsequent inflammation
Patient Presentation	Teenager or young adult presenting with facial lesions (forehead, cheeks, chin); may also involve the chest and upper back. Complaints of "pimples," "breakouts," or scarring
Classic Physical Exam	Open comedones (blackheads), closed comedones (whiteheads), inflammatory papules, pustules, nodules, or cysts distributed over sebaceous gland-rich areas (face, upper trunk)
Key Diagnostic Results	Clinical diagnosis; no labs required unless initiating isotretinoin (pregnancy test, fasting lipid panel, hepatic function panel) or suspecting hormonal etiology (total/free testosterone, DHEA-S)
Management	Mild comedonal: topical retinoid. Mild inflammatory: topical retinoid + benzoyl peroxide. Moderate: add oral antibiotic (doxycycline). Severe nodulocystic: isotretinoin
Keywords	"Comedones on the face of a teenager," "nodular cystic lesions," "iPLEDGE program," "two forms of contraception," "failed oral antibiotics"

1. Pathophysiology

Acne vulgaris develops through four interrelated mechanisms, and understanding them allows you to predict both the clinical presentation and the rationale behind each treatment.

The process begins with androgen-stimulated sebum overproduction. During puberty, rising levels of dehydroepiandrosterone sulfate (DHEA-S) and testosterone are converted to dihydrotestosterone (DHT) by 5-alpha reductase within the pilosebaceous unit. DHT binds to androgen receptors on sebocytes, driving excessive sebum secretion. This is why acne peaks during adolescence and why hyperandrogenic states such as PCOS are strongly associated with acne in adult women.

The second mechanism is follicular epithelial hyperkeratinization. Normally, keratinocytes within the follicular infundibulum shed in an orderly fashion. In acne, these keratinocytes become cohesive and accumulate, forming a plug at the follicular ostium. This plug, combined with the excess sebum behind it, creates the microcomedone, the precursor lesion of all acne. Open comedones (blackheads) form when the follicular opening dilates and the sebum oxidizes at the surface. Closed comedones (whiteheads) form when the opening remains narrow.

Third, Cutibacterium acnes (formerly Propionibacterium acnes) thrives in the anaerobic, lipid-rich environment behind the plugged follicle. This gram-positive anaerobic rod metabolizes triglycerides in sebum into free fatty acids and produces lipases, proteases, and hyaluronidases that damage the follicular wall.

Fourth, the damaged follicular wall triggers an inflammatory cascade. C. acnes activates toll-like receptor 2 (TLR-2) on keratinocytes and macrophages, leading to release of IL-1, IL-8, TNF-alpha, and other cytokines. Neutrophilic infiltration produces the papules and pustules seen clinically. When the follicular wall ruptures entirely and sebum/keratin spill into the dermis, a vigorous foreign body-type granulomatous reaction produces nodules and cysts, the hallmark of severe disease.

This pathophysiologic sequence explains why topical retinoids (targeting hyperkeratinization), benzoyl peroxide (targeting C. acnes), oral antibiotics (targeting inflammation and C. acnes), and isotretinoin (targeting all four mechanisms simultaneously) each occupy a defined role in treatment.

2. Classification and Clinical Manifestation

Severity	Lesion Types	Distribution	Notes
Mild comedonal	Open and closed comedones predominate; few or no inflammatory lesions	Face (forehead, nose, chin)	The earliest clinical form; represents follicular plugging without significant inflammation
Mild to moderate inflammatory	Papules and pustules with some comedones	Face; may extend to upper trunk	Inflammatory component now present; neutrophilic infiltration is evident
Moderate to severe inflammatory	Numerous papules, pustules, and occasional nodules	Face, chest, and back	More widespread; higher risk of scarring
Severe nodulocystic (conglobata)	Deep-seated nodules, cysts, sinus tracts, and abscesses	Face, chest, back, and shoulders	Highest risk of scarring and disfigurement; defines the indication for isotretinoin

Additional recognized variants worth knowing for exam purposes:

Variant	Key Feature
Acne fulminans	Sudden onset of hemorrhagic/ulcerative nodules with systemic symptoms (fever, polyarthralgia, leukocytosis); may require systemic corticosteroids before isotretinoin
Drug-induced acne (acneiform eruption)	Monomorphic papulopustular lesions (no comedones) appearing abruptly; associated with corticosteroids, lithium, phenytoin, isoniazid, anabolic steroids
Neonatal acne	Comedones and papules on the face of a newborn (first 6 weeks); driven by maternal and neonatal androgens; self-limited
Acne mechanica	Triggered by friction, pressure, or occlusion (helmets, chin straps, tight clothing)
Hormonal acne in adult women	Papules and deep nodules along the jawline and lower face; flares with menstrual cycle; suggests androgen excess

The distinction between true acne vulgaris (which always has comedones) and acneiform eruptions (which lack comedones and are monomorphic) is a frequently tested discriminator.

3. Diagnostic Workup

Test	Purpose	When to Order
Clinical examination	Establish diagnosis, classify severity	All patients; this is the primary diagnostic method
Pregnancy test (urine or serum beta-hCG)	Rule out pregnancy before isotretinoin	Required before and monthly during isotretinoin
Fasting lipid panel	Baseline and monitoring for hypertriglyceridemia	Before and during isotretinoin therapy
Hepatic function panel (ALT, AST)	Monitor for hepatotoxicity	Before and during isotretinoin therapy
CBC	Rarely needed; check if isotretinoin-related cytopenias suspected	During isotretinoin if clinically indicated
Total and free testosterone, DHEA-S	Evaluate for hyperandrogenism	Adult women with treatment-resistant acne, irregular menses, hirsutism, or virilization
17-hydroxyprogesterone	Screen for non-classic congenital adrenal hyperplasia (NCCAH)	When DHEA-S is elevated or clinical suspicion for CAH

Acne vulgaris is a clinical diagnosis. There is no "best initial test" or "gold standard" imaging study. The diagnosis is established by identifying the combination of comedones (open and/or closed) with or without inflammatory lesions in a characteristic distribution. The presence of comedones is essential to the diagnosis; if they are absent and the eruption is monomorphic, consider acneiform eruptions from medications or other conditions.

Laboratory testing becomes relevant in two clinical scenarios:

First, before starting isotretinoin. Because isotretinoin is a known teratogen (FDA pregnancy category X), a pregnancy test is mandatory before initiation and must be repeated monthly throughout treatment and for one month after discontinuation. A fasting lipid panel and hepatic function tests serve as baselines, since isotretinoin commonly causes dose-dependent hypertriglyceridemia and can elevate transaminases. Lipids and liver enzymes should be rechecked at 1 to 2 months and periodically thereafter.

Second, when hormonal acne is suspected. Adult women presenting with acne concentrated along the jawline and lower face, treatment-resistant acne, or concurrent signs of androgen excess (hirsutism, alopecia, oligomenorrhea) should undergo hormonal evaluation. Elevated total or free testosterone and DHEA-S suggest either PCOS or an adrenal androgen source. An elevated early-morning 17-hydroxyprogesterone points toward non-classic congenital adrenal hyperplasia, which can mimic PCOS closely.

Bacterial culture of acne lesions is not routinely indicated and should only be considered when gram-negative folliculitis is suspected (a patient on prolonged oral antibiotics who develops sudden worsening with pustules concentrated around the nose).

4. Management and Treatment

Acne Severity	First-Line Treatment	Adjuncts / Alternatives
Mild comedonal	Topical retinoid (tretinoin, adapalene)	Topical azelaic acid as alternative
Mild inflammatory	Topical retinoid + benzoyl peroxide (with or without topical antibiotic)	Benzoyl peroxide-clindamycin combination gel
Moderate inflammatory	Topical retinoid + benzoyl peroxide + oral antibiotic (doxycycline or minocycline)	Oral contraceptives or spironolactone in women
Severe nodulocystic	Isotretinoin	Intralesional triamcinolone for acute, painful nodules
Hormonal acne (women)	Combined oral contraceptive pills (OCPs) and/or spironolactone	Added to topical regimen; not used as monotherapy
Maintenance (post-clearance)	Topical retinoid with or without benzoyl peroxide	Continue long-term to prevent relapse

Topical Retinoids are the backbone of almost every acne regimen. Tretinoin (0.025% to 0.1% cream or gel), adapalene (0.1% or 0.3% gel), and tazarotene (0.05% or 0.1%) normalize follicular keratinization, prevent microcomedone formation, and have mild anti-inflammatory properties. They are applied once nightly. Patients should be counseled about initial irritation (erythema, peeling, dryness) during the first 2 to 4 weeks and photosensitivity requiring daily sunscreen use. Adapalene 0.1% is the best tolerated and is available over the counter. Retinoids are contraindicated in pregnancy (tazarotene is category X; tretinoin and adapalene are category C but generally avoided).

Benzoyl Peroxide (BPO) is a bactericidal agent that generates free oxygen radicals within the follicle, killing C. acnes without inducing antibiotic resistance. This is why BPO is always paired with topical antibiotics: it prevents the emergence of resistant organisms. Available in concentrations of 2.5% to 10%, with 2.5% to 5% being equally effective and less irritating than 10%. Applied once or twice daily.

Topical Antibiotics (clindamycin 1%, erythromycin 2%) reduce C. acnes burden and inflammation. They should never be used as monotherapy because of the risk of resistance. Always combine with BPO. Limit topical antibiotic use to 12 weeks whenever possible.

Oral Antibiotics are added for moderate inflammatory acne that is not adequately controlled with topical therapy alone. Doxycycline (50 to 100 mg twice daily or 100 mg once daily) is preferred over minocycline due to a better safety profile. Minocycline (50 to 100 mg twice daily) is an alternative but carries risks of drug-induced lupus, hyperpigmentation, and vestibular toxicity. Oral antibiotics should be used for no longer than 3 to 4 months and always in combination with a topical retinoid and BPO. Upon improvement, discontinue the oral antibiotic and continue topical maintenance.

An important contraindication to remember: do not prescribe tetracyclines to pregnant women or children under 8 years of age (tooth discoloration, impaired bone growth). Use erythromycin or azithromycin instead in these populations.

Isotretinoin (0.5 to 1 mg/kg/day, divided into twice daily dosing with food) is the only therapy that targets all four pathogenic mechanisms. It is indicated for severe nodulocystic acne, acne causing scarring, or acne refractory to adequate trials of oral antibiotics. A typical treatment course lasts 15 to 20 weeks, aiming for a cumulative dose of 120 to 150 mg/kg. Relapse rates decrease at higher cumulative doses.

Isotretinoin requires enrollment in the iPLEDGE program (a risk management program). Key requirements include:

Two negative pregnancy tests before initiation (one at qualification, one on day 2 of the next menstrual period)
Two forms of contraception used simultaneously, starting one month before therapy, throughout treatment, and for one month after
Monthly pregnancy tests during therapy
Monthly follow-up with fasting lipids and liver enzymes

Side effects that are high-yield for exams: teratogenicity (the single most important point), hypertriglyceridemia, elevated transaminases, mucocutaneous dryness (cheilitis is nearly universal), myalgias, pseudotumor cerebri (especially when combined with tetracyclines, which is why you must never use them together), and night vision changes.

Hormonal Therapy is an option for female patients. Combined oral contraceptive pills containing estrogen suppress ovarian androgen production and increase sex hormone-binding globulin (SHBG), which lowers free testosterone levels. Spironolactone (25 to 200 mg daily) acts as an androgen receptor blocker and inhibits 5-alpha reductase. Spironolactone is contraindicated in pregnancy (feminization of male fetus) and requires monitoring of potassium (risk of hyperkalemia). It should never be used in male patients due to gynecomastia and anti-androgen effects.

Intralesional corticosteroid injection (triamcinolone acetonide 2.5 to 5 mg/mL) provides rapid resolution of individual, painful, inflammatory nodules within 48 to 72 hours. This is the "next best step" for a patient presenting with a single large painful nodule when you need quick relief.

Maintenance Therapy

After clearance, a topical retinoid (with or without BPO) should be continued indefinitely to prevent relapse. This is the most commonly missed step in acne management.

5. Differential Diagnosis and Distractors

Differential	Why It Is Similar	Key Discriminator
Rosacea	Papules and pustules on the central face in adults	Rosacea has no comedones, plus there is background erythema, telangiectasias, and flushing; typically affects patients over 30
Perioral dermatitis	Papules and pustules around the mouth	Small, grouped papulovesicles with a clear zone sparing the vermilion border; often triggered by topical corticosteroids on the face
Folliculitis (bacterial)	Red papules and pustules that look like acne	Lesions are centered on hair follicles with no comedones; culture grows S. aureus; location often on trunk, buttocks, or thighs
Pityrosporum folliculitis (Malassezia)	Pruritic, monomorphic papules and pustules on the trunk	Intensely pruritic (acne rarely itches), monomorphic (uniform size), no comedones; worsens with antibiotics; responds to antifungals
Drug-induced acneiform eruption	Papules and pustules that look like acne	Abrupt onset, monomorphic lesions, absence of comedones, temporal relationship to an offending drug (corticosteroids, lithium, phenytoin)
Hidradenitis suppurativa	Nodules, abscesses, and sinus tracts resembling nodulocystic acne	Located in intertriginous areas (axillae, groin, inframammary folds) rather than the face; recurrent with sinus tract formation and "double-ended" comedones
Gram-negative folliculitis	Sudden flare of pustules in a patient already being treated for acne	Occurs after prolonged oral antibiotic therapy; pustules concentrated around the nose; culture shows gram-negative organisms (Klebsiella, Enterobacter, Proteus)

6. Traps and High-Yield Pearls

The most common way students lose points on acne questions is choosing an oral antibiotic for severe nodulocystic acne instead of isotretinoin. When the vignette describes deep nodules, cysts, scarring, or "failed multiple courses of oral antibiotics," the answer is always isotretinoin. Students hesitate because they associate isotretinoin with side effects and feel safer choosing doxycycline, but the exam rewards you for knowing the correct escalation.

A second frequent trap involves combining isotretinoin with a tetracycline. Both can cause pseudotumor cerebri (idiopathic intracranial hypertension), and using them together significantly increases that risk. If a vignette shows a patient on isotretinoin who develops headache, visual changes, and papilledema, think pseudotumor cerebri and recall this contraindicated combination.

Third, students often forget that the presence or absence of comedones is the single most useful feature for distinguishing true acne from its mimics. Rosacea, folliculitis, pityrosporum folliculitis, and drug-induced acneiform eruptions all lack comedones. If the question stem describes inflammatory papules and pustules but never mentions comedones, pause and reconsider the diagnosis.

Fourth, recognize the hormonal acne pattern in adult women: jawline and lower face distribution, premenstrual flares, and signs of hyperandrogenism. The correct next step is hormonal evaluation and the addition of spironolactone or OCPs, not simply escalating topical therapy.

Finally, always remember maintenance therapy. After successful treatment of moderate to severe acne, a topical retinoid should be continued long-term. The exam may present a patient who cleared with oral antibiotics, stopped everything, and relapsed. The teaching point is that a maintenance retinoid prevents relapse and avoids the need for repeated antibiotic courses.

The core competency being tested in acne questions is your ability to match the severity of disease to the correct tier of therapy, recognize when to escalate to isotretinoin, understand its monitoring requirements and contraindications, and distinguish acne from conditions that mimic it based on the presence or absence of comedones.

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