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Quizzes > Health & Medicine

The Surface of Healthy Skin Is Slightly Acidic: Quiz

Quick quiz to test your knowledge; instant results on why skin is slightly acidic.

Editorial: Review CompletedCreated By: Vivian TranUpdated Aug 23, 2025
Difficulty: Moderate
2-5mins
Learning OutcomesCheat Sheet
Paper art illustration showing skin layers and nutrients for a skin structure and nutrition quiz on dark blue background.

This quiz helps you check why the surface of healthy skin is slightly acidic and how the epidermis protects you. Practice key anatomy in minutes, then deepen your review with layers of the skin quiz and dermatology quiz questions, or take a quick ph quiz to refresh pH basics.

What is the ideal surface pH of healthy skin?
4.5
7.5
6.5
5.5
Healthy skin typically has a surface pH around 5.5, creating an acidic environment that protects against pathogens and supports the barrier. The acid mantle is vital for enzymatic activities in the stratum corneum. An optimal pH also maintains the skin microbiome balance.
Which layer of the epidermis is the outermost?
Stratum granulosum
Stratum basale
Stratum spinosum
Stratum corneum
The stratum corneum is the outermost epidermal layer composed of dead, flattened keratinocytes. It forms the primary barrier to water loss and pathogen entry. Its integrity is crucial for skin hydration and protection.
What is the primary cell type in the epidermis?
Fibroblasts
Langerhans cells
Melanocytes
Keratinocytes
Keratinocytes make up about 90% of epidermal cells, producing keratin and forming the barrier. They originate in the stratum basale and migrate upward to form the stratum corneum. Their differentiation is essential for skin resilience.
Which cells produce melanin in the skin?
Keratinocytes
Melanocytes
Langerhans cells
Merkel cells
Melanocytes synthesize melanin in melanosomes and transfer it to keratinocytes. Melanin protects DNA from UV radiation damage. Variation in melanocyte activity determines skin color.
Langerhans cells are primarily involved in what function?
Collagen synthesis
Immune defense
Sensory reception
Pigment production
Langerhans cells are antigen-presenting cells in the epidermis, initiating immune responses to pathogens. They capture antigens and migrate to lymph nodes to activate T cells. This surveillance helps maintain skin immunity.
What does TEWL stand for in skincare?
Total Epidermal Water Loss
Transepidermal Water Loss
Trans Epidermal Water Loss
Total Epidermal Wax Layer
Transepidermal water loss (TEWL) measures water evaporating through the epidermis, reflecting barrier integrity. Low TEWL indicates a healthy barrier, while high TEWL suggests dysfunction. It's a key parameter in dermatology studies.
The natural moisturizing factor in skin is abbreviated as:
TMF
EMF
NMF
PMF
Natural Moisturizing Factor (NMF) consists of amino acids and derivatives in the stratum corneum that attract and retain water. It's crucial for skin hydration and flexibility. Depletion of NMF leads to dryness and irritation.
Which lipid class is most abundant in the stratum corneum?
Ceramides
Free fatty acids
Cholesterol
Phospholipids
Ceramides make up around 50% of stratum corneum lipids by weight, essential for barrier function and water retention. They form lamellar structures that seal the skin. Imbalance in ceramides causes dryness and barrier defects.
What is the main function of the acid mantle?
Barrier to microbes
UV protection
Sensory detection
Collagen synthesis
The acid mantle is a thin, acidic film on the surface that inhibits pathogen growth and maintains enzyme activity for barrier homeostasis. It's key in preserving the healthy microbiome. Disruption leads to infections and irritation.
Which vitamin is essential for normal epidermal differentiation?
Vitamin A
Vitamin E
Vitamin D
Vitamin C
Vitamin A (retinoids) regulates keratinocyte proliferation and differentiation, maintaining epidermal structure. Deficiency leads to impaired barrier and dryness. Topical retinoids are used for acne and aging skin.
Which nutrient is crucial for collagen synthesis in the dermis?
Vitamin C
Vitamin K
Vitamin D
Vitamin A
Vitamin C is a cofactor for prolyl and lysyl hydroxylases in collagen synthesis, stabilizing the triple helix. Deficiency leads to scurvy with weak connective tissue. It's a potent antioxidant supporting skin health.
What is the primary role of sebum on skin?
Water loss
Lubrication and barrier support
Microbial growth
Pigmentation
Sebum, produced by sebaceous glands, lubricates skin, forms part of the acid mantle, and maintains barrier integrity. Its lipids also have mild antimicrobial properties. Overproduction can lead to acne.
Which process describes water evaporation through intact skin?
Sweating
Transdermal diffusion
Sensory transduction
Insensible perspiration
Insensible perspiration is the passive loss of water vapor through the stratum corneum, not noticeable as sweat. It reflects barrier permeability. High rates indicate barrier damage.
Which skin component acts as a waterproofing agent in the stratum corneum?
Melanosomes
Collagen fibers
Intercellular lipids
Keratin fibrils
Intercellular lipids (ceramides, cholesterol, free fatty acids) fill spaces between corneocytes, forming a hydrophobic barrier. This structure prevents water loss and entry of irritants. Maintaining lipid balance is critical for skin health.
Which epidermal layer contains the protein filaggrin that aids barrier formation?
Stratum spinosum
Stratum lucidum
Stratum granulosum
Stratum basale
Filaggrin is synthesized in the stratum granulosum as profilaggrin and processed to filaggrin, which aggregates keratin filaments. It contributes to natural moisturizing factor as it degrades. Mutations in filaggrin lead to barrier defects like atopic dermatitis.
Which enzyme family helps maintain skin acidity by generating free fatty acids?
Proteases
Lipases
Amylases
Nucleases
Epidermal lipases hydrolyze sebum triacylglycerols into free fatty acids, contributing to the acid mantle. These acids lower surface pH and support antimicrobial defense. Dysfunction can impair barrier function.
What is the approximate water content of the stratum corneum under normal conditions?
15 - 20%
5 - 10%
35 - 40%
25 - 30%
Normal stratum corneum water content ranges around 15 - 20%, critical for flexibility and barrier function. Higher or lower levels indicate hydration issues. NMF and intercellular lipids regulate this content.
Which microorganism is a common commensal on healthy skin?
Staphylococcus epidermidis
Candida albicans
Escherichia coli
Streptococcus pyogenes
Staphylococcus epidermidis is a predominant skin commensal that helps prevent pathogen colonization. It produces antimicrobial peptides and supports immune balance. Dysbiosis can lead to infections.
Which essential fatty acid must be obtained through diet for healthy skin barrier lipids?
Stearic acid
Palmitic acid
Oleic acid
Linoleic acid
Linoleic acid (omega-6) is essential for ceramide synthesis in the stratum corneum. Dietary deficiency impairs barrier function and increases TEWL. It's found in sunflower and safflower oils.
Which vitamin derivative is often used topically to normalize differentiation in acne treatment?
Retinol
Ascorbic acid
Tretinoin
Niacinamide
Tretinoin (all-trans retinoic acid) modulates gene expression to promote normal keratinocyte turnover and reduce comedones. It also improves collagen production. It's a standard acne and photoaging therapy.
Which stratum is present only in thick skin like palms and soles?
Stratum basale
Stratum granulosum
Stratum spinosum
Stratum lucidum
Stratum lucidum is a translucent layer found only in thick skin areas, between the granulosum and corneum. It contains eleidin, a keratin intermediate. This layer adds extra protection where friction is high.
Which molecule acts as a pH buffer in the stratum corneum?
Elastin
Urea
Cholesterol
Keratin
Urea, part of NMF, helps buffer skin pH by attracting and retaining water and modulating acid - base balance. It also supports enzymatic activities for desquamation. Deficiency relates to dryness.
What is the main function of tight junctions in the epidermis?
Melanin transfer
Mechanical support
Preventing paracellular water loss
Cell signaling
Tight junctions in the granular layer seal gaps between keratinocytes, preventing uncontrolled water and ion loss through paracellular routes. They complement lipid barriers. Dysfunction elevates TEWL.
Niacinamide in topical formulations primarily supports:
Melanin production
Protein denaturation
Barrier repair and anti-inflammatory effects
Excess sebum secretion
Niacinamide enhances lipid synthesis, reduces TEWL, and has anti-inflammatory properties, improving barrier function and reducing redness. It also inhibits melanosome transfer, lightening hyperpigmentation.
Which keratinocyte receptor senses external chemical irritants to trigger inflammation?
PDGFR
FGFR
EGFR
TRPV1
TRPV1 (transient receptor potential vanilloid 1) on keratinocytes responds to heat and chemical irritants, leading to calcium influx and release of proinflammatory cytokines. It contributes to itch and pain. Antagonists are explored for inflammatory skin conditions.
A deficiency in which amino acid impairs NMF production?
Glycine
Histidine
Valine
Leucine
Histidine is a key precursor of urocanic acid and part of the NMF pool, contributing to hydration and UV protection. Low histidine reduces NMF levels and barrier function.
Which diagnosis is associated with a mutated filaggrin gene?
Rosacea
Vitiligo
Psoriasis
Atopic dermatitis
Filaggrin gene (FLG) mutations disrupt barrier formation, leading to dry, inflamed skin typical of atopic dermatitis. Carriers often have ichthyosis vulgaris. Proper filaggrin function is key to barrier integrity.
Which ion transporter contributes to acidifying the stratum corneum?
KCC1 (K+/Cl? co-transporter)
NHE1 (Na+/H+ exchanger)
ENaC (Na+ channel)
CFTR (Cl? channel)
NHE1 exchanges intracellular Na+ for extracellular H+, releasing protons into the stratum corneum and contributing to acid mantle maintenance. This gradient supports enzyme activities for barrier homeostasis. NHE1 dysfunction raises skin pH.
Which epidermal protease is activated at acidic pH for desquamation?
Elastase
Kallikrein 5
Matrix metalloproteinase-1
Cathepsin D
Kallikrein 5 is a serine protease in the stratum corneum that cleaves corneodesmosomes at acidic pH, enabling desquamation. Its activity is tightly regulated by pH and inhibitors. Overactivity causes barrier disruption.
Which ceramide subclass is most abundant in human stratum corneum?
Ceramide AP
Ceramide EOS (?-OH acyl-sphingosine)
Ceramide NP
Ceramide NS
Ceramide EOS, with an ?-hydroxy fatty acid linked to sphingosine, is predominant in the stratum corneum, crucial for forming long periodicity lamellar phases. It's essential for barrier lipid organization.
Which pathway synthesizes ceramides in keratinocytes?
Salvage pathway only
Phosphatidylcholine pathway
Sphingomyelinase pathway only
De novo pathway
The de novo pathway in the ER synthesizes ceramides from serine and palmitoyl-CoA via serine palmitoyltransferase, then dihydroceramide desaturase. Salvage and sphingomyelinase pathways also contribute. Full function requires all pathways.
Which tight junction protein is critical in the granular layer?
E-cadherin
Beta-catenin
Claudin-1
Occludin
Claudin-1 is a major tight junction protein in the stratum granulosum, essential for sealing the paracellular barrier. Knockout models display severe water loss and barrier defects. It's a key regulator of epidermal integrity.
Which amino acid derivative in NMF provides UV protection?
Lactic acid
Histamine
Urocanic acid
Urea
Trans-urocanic acid, derived from histidine in the stratum corneum, absorbs UV-B and contributes to photoprotection. It also modulates local immune responses. Isomerization to cis form affects immunosuppression.
Which gluco-ceramide participates in barrier lipid organization?
Sphingomyelin
Lactosylceramide
Phosphatidylserine
Glucosylceramide
Glucosylceramides in lamellar granules are hydrolyzed to ceramides by ?-glucocerebrosidase in the stratum corneum, essential for lamellar structure. Mutations cause barrier diseases like Gaucher's.
Which transporter moves cholesterol into lamellar granules?
NPC1
ABCA12
ABCG1
SR-BI
ABCA12 transports lipids including glucosylceramides and cholesterol into lamellar granules in keratinocytes. Mutations lead to harlequin ichthyosis with severe barrier loss. It's vital for skin lipid secretion.
Which pH-sensitive enzyme regulates corneocyte desmosome degradation?
Granzyme B
Cathepsin D
KLK7
SASPase
SASPase (skin aspartic protease) activates at acidic pH in the stratum corneum, processing profilaggrin and facilitating desmosome degradation during desquamation. Its activity supports corneocyte shedding.
Which lipid processing defect causes Netherton syndrome?
ABCA12 mutation
FLG mutation
SPINK5 mutation
LAMTOR2 mutation
Netherton syndrome stems from SPINK5 mutations encoding the LEKTI protease inhibitor, leading to unregulated kallikrein activity and barrier breakdown. Patients have high TEWL and severe ichthyosis.
Which acid in NMF adjusts skin pH upon UV exposure?
Lactic acid
Pyruvic acid
Trans-urocanic acid
Fatty acid
Trans-urocanic acid absorbs UV-B, isomerizes to cis-form, affecting local pH and immune modulation. It helps maintain skin acid mantle after UV stress. This photoproduct links NMF to photoprotection.
Which proton pump contributes to acid mantle formation in the epidermis?
Ca2+ ATPase
H+/K+ ATPase
V-ATPase
Na+/K+ ATPase
V-ATPase in keratinocytes extrudes protons into extracellular spaces in the stratum granulosum and corneum, contributing to acid mantle and pH gradient. It works alongside NHE1. Inhibition disrupts barrier homeostasis.
Which lipid remodeling enzyme in the epidermis is pH-dependent and crucial for stratum corneum structure?
?-Glucocerebrosidase
Phospholipase D
Lipid phosphate phosphatase
Acid sphingomyelinase
?-Glucocerebrosidase is active at acidic pH in the stratum corneum, converting glucosylceramides to ceramides and glucose, essential for lamellar organization. Defects lead to Gaucher disease skin manifestations.
Which neurotransmitter-like molecule in the skin modulates pH and barrier repair?
Calcitonin gene-related peptide
Acetylcholine
Dopamine
Substance P
Calcitonin gene-related peptide (CGRP) is released from sensory fibers in response to pH shifts and injury, promoting barrier repair and modulating local immunity. It influences cytokine release and keratinocyte proliferation.
Which skin enzyme generates free amino acids from filaggrin at acidic pH?
Peptidylarginine deiminase
Granzyme A
Bleomycin hydrolase
Elastase
Bleomycin hydrolase in the stratum corneum cleaves filaggrin into free amino acids under acidic conditions, contributing to NMF and hydration. Its activity is pH-regulated.
Which microRNA is implicated in regulating epidermal lipid synthesis and pH homeostasis?
miR-203
miR-146a
miR-21
miR-155
miR-203 is highly expressed in keratinocytes and regulates genes involved in lipid synthesis, differentiation, and pH regulation. Dysregulation affects barrier function. Studies show its role in skin homeostasis.
Which gene regulator senses acidic pH to control barrier repair gene expression?
AP-1
HIF-1?
NF-?B
Nrf2
Nrf2 activation by acid-induced oxidative signals upregulates antioxidant and barrier repair genes in keratinocytes. It maintains redox balance and lipid production. Knockout models have delayed barrier recovery.
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Study Outcomes

  1. Understand skin surface pH -

    Explain why the surface of healthy skin is slightly acidic and how this pH balance supports barrier function and microbial defense.

  2. Describe epidermis structure -

    Identify key layers and cells of the epidermis, drawing on what is true of the epidermis Milady guidelines to outline its protective roles.

  3. Analyze stratum corneum function -

    Assess what is true of the stratum corneum Milady definitions, including its contribution to hydration and barrier maintenance.

  4. Apply skin nutrition principles -

    Demonstrate how nutrients influence skin health by linking dietary factors to epidermal regeneration and surface acidity.

  5. Integrate Milady terminology -

    Use healthy skin is Milady concepts accurately to reinforce professional communication about skin structure and function.

  6. Evaluate quiz performance -

    Review quiz results to identify knowledge gaps and strengthen understanding of skin surface pH, epidermis facts, and nutrition principles.

Cheat Sheet

  1. Acid Mantle Balance -

    The surface of healthy skin is slightly acidic, typically between pH 4.5 and 5.5, forming the acid mantle that wards off harmful microbes (Journal of Dermatological Science). Mnemonic: "A for Acid Mantle, A for Antimicrobial." This slight acidity also optimizes enzymes that support natural exfoliation and barrier repair.

  2. Avascular Epidermis -

    What is true of the epidermis milady? It is avascular, meaning it has no blood vessels and relies on diffusion from the dermis for nutrients (American Academy of Dermatology). This design keeps the outer layer lightweight and allows keratinocytes to migrate upward as they mature.

  3. Stratum Corneum Barrier -

    What is true of the stratum corneum milady? As the outermost layer, it consists of dead, flattened keratinocytes embedded in lipids that form the primary barrier against water loss and pathogens (International Journal of Cosmetic Science). Think "bricks and mortar," where the cells are bricks and intercellular lipids are mortar.

  4. Natural Moisturizing Factor (NMF) -

    NMF is a mix of amino acids, PCA, urea, and lactic acid that attracts and holds water in the stratum corneum (Skin Pharmacology and Physiology). Remember the formula pH = - log[H❺] to appreciate why NMF needs an acidic environment to remain effective.

  5. Nutrition & Collagen Synthesis -

    Healthy skin is Milady-approved when you fuel it with vitamin C, essential fatty acids, and antioxidants that support collagen production and barrier integrity (National Institutes of Health). A simple tip: pair citrus fruits with healthy fats to boost vitamin C absorption and skin resilience.

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