A 21-year-old man comes to the physician because of painful, firm, dark bumps on his neck and jawline. He has no history of serious illness and takes no medications. His brother had a similar rash. A photograph of the rash is shown. Which of the following is the most likely underlying mechanism of this patient's condition?

Interfollicular penetration of the skin by distal end of hair

Trichophyton infection of the superficial hair follicle

Bacterial infection of the superficial or deep hair follicle

Follicular obstruction with subsequent duct rupture

Cutibacterium acnes colonization of the pilosebaceous unit

A 41-year-old male who takes NSAIDs regularly for his chronic back pain develops severe abdominal pain worse with eating. Upper endoscopy is performed and the medical student asks the supervising physician how the histological differentiation between a gastric ulcer and erosion is made. Which of the following layers of the gastric mucosa MUST be breached for a lesion to be considered an ulcer?

Epithelium, lamina propria, muscularis mucosa

Epithelium, lamina propria, muscularis mucosa, submucosa, and adventitia

Epithelium, lamina propria, muscularis mucosa, and submucosa

A 15-year-old boy comes to the physician because of skin changes on his face, chest, and back over the past year. Treatment with over-the-counter benzoyl peroxide has been ineffective. Physical examination shows numerous open comedones, inflammatory papules, and pustules on his face, chest, and back. Which of the following is the most likely underlying mechanism of this patient’s skin condition?

Hyperkeratinization of hair follicles

Type IV hypersensitivity reaction

Formation of superficial epidermal inclusion cyst

Hyperplasia of pilosebaceous glands

A 9-year-old boy with a history of acute lymphoblastic leukemia is brought to the clinic by his mother because of pruritic vesicles that appeared on the left side of his torso 12 hours ago. One day earlier, before the appearance of the vesicles, the patient's mother notes that he had been complaining of a burning sensation in that area. The boy has been receiving chemotherapy consisting of methotrexate, cytarabine, and cyclophosphamide for 1 month. He received the last treatment 2 days ago. He has no other past medical history. The patient is afebrile and vital signs are within normal limits. Upon physical examination, painful vesicles are localized to the left C7 skin dermatome (see image). Which of the following is the most likely etiology of the skin lesions in this patient?

Reactivation of VZV due to immunodeficiency caused by chemotherapy

Bacterial infection of the skin

Reactivation of the varicella zoster virus (VZV) due to congenital immunodeficiency

A 35-year-old man is brought to the emergency department from a kitchen fire. The patient was cooking when boiling oil splashed on his exposed skin. His temperature is 99.7°F (37.6°C), blood pressure is 127/82 mmHg, pulse is 120/min, respirations are 12/min, and oxygen saturation is 98% on room air. He has dry, nontender, and circumferential burns over his arms bilaterally, burns over the anterior portion of his chest and abdomen, and tender spot burns with blisters on his shins. A 1L bolus of normal saline is administered and the patient is given morphine and his pulse is subsequently 80/min. A Foley catheter is placed which drains 10 mL of urine. What is the best next step in management?

Additional fluids and escharotomy

Additional fluids and admission to the ICU

An otherwise healthy 17-year-old girl comes to the physician because of multiple patches on her face, hands, abdomen, and feet that are lighter than the rest of her skin. The patches began to appear 3 years ago and have been gradually increasing in size since. There is no associated itchiness, redness, numbness, or pain. She emigrated from India 2 years ago. An image of the lesions on her face is shown. Which of the following is most likely involved in the pathogenesis of this patient's skin findings?

Autoimmune destruction of melanocytes

Infection with Malassezia globosa

Infection with Mycobacterium leprae

Integumentary system — USMLE Lesson

Layers of the Skin - The Body's Armor

Skin Layers Cross-Section with Hair Follicles and Glands

Epidermis: Avascular, keratinized stratified squamous epithelium. Layers from superficial to deep:
- S. Corneum: Anucleated cells, keratin.
- S. Lucidum: Clear layer; only in thick skin (palms/soles).
- S. Granulosum: Keratohyalin granules.
- S. Spinosum: Desmosomes (intercellular bridges).
- S. Basale: Mitotically active stem cells, melanocytes.
- 📌 Mnemonic: Come, Let's Get Sun Burned.
Dermis: Supports epidermis; contains blood vessels, nerves, and appendages.
- Papillary Layer: Loose connective tissue.
- Reticular Layer: Dense, irregular connective tissue.

⭐ Langerhans cells (dendritic, antigen-presenting immune cells) are most prominent in the Stratum Spinosum.

Cells of the Epidermis - The Skin's Crew

Keratinocytes (~90%): The workhorses. Originate in the basal layer, mature upwards. Produce keratin for strength and form the waterproof barrier.
Melanocytes (~5%): Located in the stratum basale. Synthesize melanin pigment from tyrosine, protecting against UV radiation. Derived from the neural crest.
Langerhans Cells (2-3%): Dendritic cells in the stratum spinosum. Act as antigen-presenting cells (APCs). 📌 Birbeck granules (tennis-racket shape) are characteristic.
Merkel Cells (~1%): Found in the stratum basale. Slow-adapting mechanoreceptors for light touch.

⭐ Albinism results from a normal number of melanocytes but a lack of tyrosinase, the key enzyme for melanin production.

Microscopic view of epidermis and dermis layers

Skin Appendages - Hairy, Sweaty, Oily Bits

Anatomy of Hair Follicle and Associated Structures

Hair Follicles
- Anchors hair shaft; associated with arrector pili muscles (piloerection).
- Cycles: Anagen (growth), Catagen (involution), Telogen (rest).
Sebaceous Glands
- Produce oily/waxy sebum; lubricates skin & hair.
- Holocrine secretion: whole cell disintegrates.
- Outflow is typically into a hair follicle.
Sweat (Sudoriferous) Glands
- Eccrine: Everywhere; for thermoregulation. Secretes watery sweat via merocrine secretion.
- Apocrine: Axillae, genitals. Active at puberty. Viscous, odorless fluid that bacteria make odorous.

⭐ High-Yield: Despite the name, apocrine sweat glands also use merocrine secretion (exocytosis), not apocrine secretion (decapitation). This is a common exam trap!

Dermatomes & Innervation - The Nerve Map

Dermatome map with spinal nerve levels and spinal cord

A dermatome is an area of skin supplied by sensory fibers from a single spinal nerve (dorsal root).
Key anatomical landmarks for testing:
- C4: High collar shirt (clavicles/upper shoulders)
- T4: Nipple line (teat pore)
- T10: Umbilicus (belly but-ten)
- L4: Kneecaps (down on all L4s)
- S2-S4: Perineal sensation

⭐ Herpes zoster (shingles) infection causes a painful, unilateral vesicular rash that is limited to a single dermatome and does not cross the midline.

Burns & Healing - Trial by Fire

Classification (Depth):
- 1st: Epidermis only (e.g., sunburn); erythema, painful.
- 2nd: Epidermis + Dermis; blistering, very painful.
- 3rd: Full-thickness; white/leathery, painless.
- 4th: Extends to fascia, muscle, or bone.
Severity: Total Body Surface Area (TBSA) via "Rule of Nines."
Healing Phases:

⭐ Curling's Ulcer: Acute gastric stress ulcer in proximal duodenum post-severe burn. Caused by ↓ plasma volume → ischemia of gastric mucosa.

High‑Yield Points - ⚡ Biggest Takeaways

The epidermis is avascular and its layers can be recalled with the mnemonic: Come, Let's Get Sun Burned.

Neural crest cells are the origin for both melanocytes (in the stratum basale) and Merkel cells (sensory receptors).

Langerhans cells, found in the stratum spinosum, are bone marrow-derived antigen-presenting cells.

Pemphigus vulgaris (anti-desmosome) vs. Bullous pemphigoid (anti-hemidesmosome) are key autoimmune blistering diseases.

Third-degree burns are painless because they destroy nerve endings.

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