Phototherapy and Photobiology — MCQs
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Which of the following is NOT a complication of PUVA therapy?
A 12-year-old boy, after spending his holiday on a beach, develops pruritic hemorrhagic vesicles on his cheeks, ears, nose, and hands 12 hours after sun exposure. A week later, the lesions crusted and healed with permanent scars. What is the most probable diagnosis?
Psoralen plus ultraviolet A (PUVA) therapy is useful in which of the following conditions?
Psoralen is used in the treatment of:
What is the wavelength range of UVB rays?
What is the range of light that causes maximum skin damage?
Circulating lymphocytes are most sensitive to which type of ultraviolet radiation?
A patient presents with skin bullae on sun exposure. What is the underlying defect?
A 40-year-old farmer with a history of recurrent attacks of porphyria complains of itching when exposed to the sun and a maculopapular rash on sun-exposed areas. His symptoms are exaggerated in the summer. What is the diagnosis?
All of the following are true about solar urticaria, EXCEPT:
Phototherapy and Photobiology Indian Medical PG Practice Questions and MCQs
Question 1: Which of the following is NOT a complication of PUVA therapy?
- A. Premature aging of the skin
- B. Cataracts
- C. Skin cancers
- D. Exfoliative dermatitis (Correct Answer)
Explanation: **Explanation:** PUVA (Psoralen + Ultraviolet A) therapy involves the administration of a photosensitizer (8-methoxypsoralen) followed by exposure to UVA radiation. While it is an effective treatment for conditions like psoriasis and vitiligo, it carries specific long-term and short-term risks. **Why Exfoliative Dermatitis is the correct answer:** Exfoliative dermatitis (Erythroderma) is **not** a direct complication of PUVA. In fact, PUVA is often used as a *treatment* modality for certain types of exfoliative dermatitis, such as those caused by Mycosis Fungoides or Psoriasis. While PUVA can cause a "PUVA itch" or a phototoxic burn (erythema), it does not typically trigger generalized exfoliation. **Analysis of Incorrect Options:** * **Premature aging of the skin (Dermatoheliosis):** Chronic UVA exposure leads to the degradation of collagen and elastin fibers, resulting in wrinkles, lentigines, and telangiectasia. * **Cataracts:** Psoralens distribute to the lens of the eye. If the eyes are not protected with UVA-blocking sunglasses for 24 hours post-ingestion, UVA exposure can lead to lens opacification. * **Skin cancers:** PUVA is mutagenic. Long-term therapy significantly increases the risk of Non-Melanoma Skin Cancers (NMSC), particularly **Squamous Cell Carcinoma (SCC)**. **High-Yield Clinical Pearls for NEET-PG:** * **Most common acute side effect:** Erythema (phototoxicity) and pruritus. * **Most common long-term risk:** Squamous Cell Carcinoma (SCC) is more common than Basal Cell Carcinoma (BCC) in PUVA patients (reversing the usual ratio). * **PUVA Lentigines:** Distinctive, irregular pigmented macules that appear after chronic therapy. * **Contraindications:** Pregnancy, lactation, history of skin cancer (Xeroderma Pigmentosum), and severe hepatic/renal failure.
Question 2: A 12-year-old boy, after spending his holiday on a beach, develops pruritic hemorrhagic vesicles on his cheeks, ears, nose, and hands 12 hours after sun exposure. A week later, the lesions crusted and healed with permanent scars. What is the most probable diagnosis?
- A. Polymorphic light eruption
- B. Hydroa vacciniforme (Correct Answer)
- C. Actinic prurigo
- D. Persistent light reaction
Explanation: **Explanation:** The clinical presentation of a young boy with **hemorrhagic vesicles** on sun-exposed areas (cheeks, ears, nose, hands) that heal with **permanent scarring** (varioliform scars) is pathognomonic for **Hydroa vacciniforme (HV)**. **Why Hydroa vacciniforme is correct:** HV is a rare, chronic photodermatosis primarily affecting children. It is triggered by UVA radiation. The hallmark is the progression from erythema to vesicles/bullae, which become umbilicated and hemorrhagic, eventually forming necrotic crusts. The defining feature for NEET-PG is the healing process, which results in **depressed, "vacciniform" (smallpox-like) scars**. It is often associated with **Epstein-Barr Virus (EBV)** infection. **Why other options are incorrect:** * **Polymorphic Light Eruption (PMLE):** The most common photodermatosis. While it causes pruritic papules or vesicles, it **never heals with scarring**. * **Actinic Prurigo:** A variant of PMLE common in Native Americans. It presents with intensely pruritic, excoriated papules and nodules, often involving the lips (cheilitis) and conjunctiva, but does not typically present with hemorrhagic vesicles and varioliform scarring. * **Persistent Light Reaction:** Now classified under Chronic Actinic Dermatitis. It is an eczematous reaction seen in elderly males, where skin remains sensitive to light even without allergen exposure. **High-Yield Clinical Pearls for NEET-PG:** * **Action Spectrum:** UVA is the primary trigger for HV. * **Association:** Severe, systemic cases of HV are linked to **EBV-associated T-cell lymphoproliferative disorders**. * **Differential Diagnosis:** Must be distinguished from Erythropoietic Protoporphyria (EPP), which presents with immediate burning pain and waxy scarring, but lacks the hemorrhagic bullae of HV. * **Management:** Strict photoprotection; severe cases may require antimalarials or immunosuppressants.
Question 3: Psoralen plus ultraviolet A (PUVA) therapy is useful in which of the following conditions?
- A. Vitiligo
- B. Mycosis fungoides
- C. Psoriasis
- D. All of the above (Correct Answer)
Explanation: **Explanation:** **PUVA (Psoralen + UVA)** therapy involves the administration of a photosensitizing agent (8-Methoxypsoralen) followed by exposure to long-wave ultraviolet A light (320–400 nm). The mechanism involves the formation of DNA photo-adducts, which inhibit DNA synthesis and induce apoptosis of hyperproliferating cells and T-lymphocytes. **Why "All of the Above" is Correct:** * **Psoriasis:** PUVA is a classic treatment for moderate-to-severe plaque psoriasis. It reduces the rapid turnover of keratinocytes and suppresses the local cutaneous immune response. * **Vitiligo:** Psoralens stimulate the migration and proliferation of melanocytes from the hair follicle reservoir to the depigmented skin, promoting repigmentation. * **Mycosis Fungoides (MF):** As a cutaneous T-cell lymphoma, MF is highly sensitive to the phototoxic effects of PUVA, which induces apoptosis in malignant T-cells infiltrating the epidermis. **Clinical Pearls for NEET-PG:** 1. **Mechanism:** Psoralens intercalate into DNA; UVA then causes **Type I (oxygen-independent)** reactions forming monoadducts/cross-links and **Type II (oxygen-dependent)** reactions forming free radicals. 2. **Dosage:** Oral psoralen is usually given **0.6 mg/kg**, 2 hours before UVA exposure. 3. **Side Effects:** Acute side effects include nausea and erythema. Long-term risks include **PUVA lentigines** and an increased risk of **Squamous Cell Carcinoma (SCC)**. 4. **Contraindication:** PUVA is contraindicated in patients with Xeroderma Pigmentosum, Lupus Erythematosus, and pregnancy. 5. **Current Trend:** Narrowband UVB (311 nm) has largely replaced PUVA for psoriasis and vitiligo due to a better safety profile, but PUVA remains superior for thick plaques and MF.
Question 4: Psoralen is used in the treatment of:
- A. Pemphigus
- B. Vitiligo (Correct Answer)
- C. Pityriasis alba
- D. Ichthyosis
Explanation: **Explanation:** **Psoralen** is a photosensitizing agent used in **PUVA (Psoralen + Ultraviolet A)** therapy. It belongs to the furocoumarin family and works by intercalating into DNA. Upon exposure to UVA light, it forms DNA cross-links, which inhibits keratinocyte proliferation and induces melanocyte stimulation. **Why Vitiligo is correct:** In Vitiligo, PUVA therapy is a classic treatment modality. It works by: 1. **Immunomodulation:** Suppressing the T-cell mediated destruction of melanocytes. 2. **Melanocyte Stimulation:** Promoting the migration of melanocytes from the hair follicle reservoir to the depigmented skin, leading to repigmentation. **Why other options are incorrect:** * **Pemphigus:** This is an autoimmune bullous disorder treated primarily with systemic corticosteroids and immunosuppressants (e.g., Rituximab, Azathioprine), not phototherapy. * **Pityriasis alba:** This is a mild form of dermatitis common in children, usually associated with atopy. It is treated with emollients and low-potency topical steroids; psoralens are not indicated. * **Ichthyosis:** This is a genetic disorder of keratinization characterized by fish-like scales. Treatment involves keratolytics (e.g., urea, lactic acid) and systemic retinoids. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Psoralens (e.g., 8-Methoxypsoralen) bind to pyrimidine bases (thymine) in DNA. * **Common Indications for PUVA:** Psoriasis (most common), Vitiligo, Mycosis Fungoides (CTCL), and Alopecia Areata. * **Side Effects:** Acute side effects include nausea and polymorphic light eruption. Long-term risks include **PUVA lentigines** and an increased risk of **Squamous Cell Carcinoma (SCC)**. * **Contraindication:** PUVA is contraindicated in patients with Xeroderma Pigmentosum, SLE, or a history of skin cancer.
Question 5: What is the wavelength range of UVB rays?
- A. 290 - 320 nm (Correct Answer)
- B. 320 - 360 nm
- C. 360 - 390 nm
- D. 390 - 420 nm
Explanation: **Explanation:** The electromagnetic spectrum of ultraviolet (UV) radiation is divided into three bands based on wavelength and biological activity: UVA, UVB, and UVC. **1. Why Option A is Correct:** **UVB (290–320 nm)** is known as the "erythemal" or "sunburn" spectrum. It is the range responsible for delayed tanning, vitamin D synthesis, and the development of skin cancers (basal and squamous cell carcinomas). In clinical practice, **Narrowband UVB (311–313 nm)** is the gold standard for treating conditions like vitiligo and psoriasis due to its high efficacy and lower carcinogenic risk compared to broadband. **2. Analysis of Incorrect Options:** * **Options B & C (320–400 nm):** These fall within the **UVA** range. UVA is further divided into UVA2 (320–340 nm) and UVA1 (340–400 nm). UVA is responsible for immediate tanning and photoaging (dermatoheliosis) as it penetrates deeper into the dermis. * **Option D (390–420 nm):** This range transitions from the long-wave UVA spectrum into **Visible Light** (starting at approximately 400 nm). **3. High-Yield Clinical Pearls for NEET-PG:** * **UVC (200–290 nm):** Shortest wavelength, highest energy; mostly absorbed by the ozone layer. Used in germicidal lamps. * **Minimal Erythema Dose (MED):** The smallest dose of UV radiation that produces confluent erythema at 24 hours. UVB is 1000 times more erythrogenic than UVA. * **Photoaging:** Primarily caused by UVA (A for Aging). * **Phototherapy:** PUVA (Psoralen + UVA) uses the 320–400 nm range, while NB-UVB uses 311 nm.
Question 6: What is the range of light that causes maximum skin damage?
- A. 360-400 nm
- B. 290-360 nm (Correct Answer)
- C. 240-290 nm
- D. 760-800 nm
Explanation: **Explanation:** The spectrum of light responsible for the majority of skin damage encompasses **Ultraviolet B (UVB: 290–320 nm)** and **Ultraviolet A (UVA: 320–400 nm)**. The range **290–360 nm** is the most clinically significant because it includes the entirety of UVB—the most erythemogenic and carcinogenic wavelengths—and the "short-wave" UVA (UVA2) portion. * **UVB (290–320 nm):** Known as the "burning rays," these have high energy and are primarily responsible for DNA damage (pyrimidine dimers), sunburn (erythema), and skin cancers (BCC, SCC, and Melanoma). * **UVA (320–400 nm):** Known as "aging rays," these penetrate deeper into the dermis, causing photoaging and indirect DNA damage via reactive oxygen species (ROS). **Analysis of Incorrect Options:** * **A (360–400 nm):** This is UVA1 (long-wave UVA). While it contributes to photoaging and tanning, it is significantly less potent in causing acute biological damage or DNA mutations compared to the 290–360 nm range. * **C (240–290 nm):** This falls within the UVC range. While highly energetic and dangerous, UVC is almost entirely absorbed by the Earth’s ozone layer and does not reach the skin under normal conditions. * **D (760–800 nm):** This is part of the Near-Infrared/Visible light spectrum. These wavelengths primarily produce heat and do not possess enough energy to cause direct photochemical DNA damage. **High-Yield Clinical Pearls for NEET-PG:** * **Minimal Erythema Dose (MED):** The smallest dose of UV radiation that produces confluent erythema at 24 hours. It is used to calibrate phototherapy. * **Action Spectrum:** The specific wavelength of light that produces a particular biological response (e.g., 290–300 nm is the peak for erythema). * **Narrowband UVB (NBUVB):** 311–313 nm. This is the "Gold Standard" for treating Psoriasis and Vitiligo as it maximizes therapeutic effect while minimizing burning.
Question 7: Circulating lymphocytes are most sensitive to which type of ultraviolet radiation?
- A. UV-A
- B. UV-B
- C. UV-C (Correct Answer)
- D. 760-800 nm wavelength light
Explanation: **Explanation:** The correct answer is **UV-C (200–280 nm)**. **1. Why UV-C is correct:** The sensitivity of cells to ultraviolet radiation is primarily determined by the absorption spectrum of DNA and nuclear proteins. DNA has a peak absorption at approximately **260 nm**, which falls squarely within the UV-C range. Circulating lymphocytes are highly susceptible to DNA damage and subsequent apoptosis when exposed to these short wavelengths. While UV-C from sunlight is filtered out by the Earth’s ozone layer and does not reach the skin surface, it is used clinically in **Extracorporeal Photopheresis (ECP)**. In ECP, lymphocytes are treated outside the body; UV-C is the most potent inducer of lymphocyte apoptosis among the UV spectrum. **2. Why other options are incorrect:** * **UV-A (320–400 nm):** These longer wavelengths penetrate deeper into the dermis but have low energy. They primarily cause oxidative stress rather than direct DNA breaks. While used in PUVA therapy, UV-A is significantly less "cytotoxic" to lymphocytes than UV-C. * **UV-B (280–320 nm):** UV-B causes direct DNA damage (pyrimidine dimers) and is responsible for sunburn. While it can affect lymphocytes in the superficial dermis, its energy levels and DNA-absorption efficiency are lower than UV-C. * **760–800 nm:** This range corresponds to **Infrared/Visible light**. These wavelengths do not possess enough energy to cause the photochemical reactions required to damage lymphocyte DNA. **High-Yield Clinical Pearls for NEET-PG:** * **Action Spectrum:** The specific wavelength that produces a biological response (e.g., 260 nm for DNA damage). * **Extracorporeal Photopheresis (ECP):** Used for Cutaneous T-cell Lymphoma (CTCL/Sezary Syndrome) and Graft-vs-Host Disease (GVHD). * **Minimal Erythema Dose (MED):** Most commonly associated with **UV-B** (the "burning" rays). * **Grotthus-Draper Law:** Light must be absorbed by a molecule (chromophore) to exert a biological effect.
Question 8: A patient presents with skin bullae on sun exposure. What is the underlying defect?
- A. Sugar changes
- B. DNA methylation
- C. Thymidine dimers (Correct Answer)
- D. Trinucleotide repeats
Explanation: The clinical presentation of skin bullae (blistering) upon sun exposure, especially in the context of conditions like **Xeroderma Pigmentosum (XP)**, is rooted in the body's inability to repair ultraviolet (UV) light-induced damage. ### Explanation of the Correct Answer When skin is exposed to UV radiation (specifically UVB), it causes a photochemical reaction where two adjacent pyrimidine bases (most commonly **Thymidine**) on a DNA strand become covalently linked. This creates **Thymidine dimers** (cyclobutane pyrimidine dimers). In healthy individuals, these are repaired via **Nucleotide Excision Repair (NER)**. A defect in this repair mechanism leads to accumulated mutations, cell death (manifesting as bullae/blisters), and a high risk of skin malignancies. ### Why Other Options are Incorrect * **A. Sugar changes:** UV radiation primarily targets the nitrogenous bases of DNA, not the pentose sugar-phosphate backbone. * **B. DNA methylation:** This is an epigenetic mechanism involved in gene silencing. While it plays a role in cancer, it is not the primary acute lesion caused by UV light. * **D. Trinucleotide repeats:** These are associated with dynamic mutations seen in neurological disorders like Huntington’s disease or Fragile X syndrome, not photodermatoses. ### High-Yield Clinical Pearls for NEET-PG * **Xeroderma Pigmentosum:** The classic "prototype" disease for this defect. Inherited as Autosomal Recessive. * **Action Spectrum:** UVB (290–320 nm) is the most responsible for thymidine dimer formation. * **Key Enzyme:** The defect often lies in **UV-specific endonuclease**, which is required for the initial "incising" step of NER. * **Malignancy Risk:** Patients have a >1000-fold increased risk of Basal Cell Carcinoma (BCC), Squamous Cell Carcinoma (SCC), and Melanoma.
Question 9: A 40-year-old farmer with a history of recurrent attacks of porphyria complains of itching when exposed to the sun and a maculopapular rash on sun-exposed areas. His symptoms are exaggerated in the summer. What is the diagnosis?
- A. Seborrheic dermatitis
- B. Contact dermatitis
- C. Psoriasis
- D. Porphyria cutanea tarda (Correct Answer)
Explanation: **Explanation** **Porphyria Cutanea Tarda (PCT)** is the most common type of porphyria. It results from a deficiency of the enzyme **uroporphyrinogen decarboxylase (UROD)**. The clinical presentation in this case—a farmer (chronic sun exposure) with a history of porphyria and symptoms worsening in the summer—is classic for PCT. The underlying pathophysiology involves the accumulation of porphyrins in the skin. When exposed to long-wave ultraviolet light (UVA), these porphyrins generate reactive oxygen species, leading to **photosensitivity**, skin fragility, and the characteristic maculopapular rash, vesicles, or bullae on sun-exposed areas (dorsum of hands, face). **Why other options are incorrect:** * **Seborrheic Dermatitis:** Typically affects "seborrheic" areas rich in oil glands (scalp, eyebrows, nasolabial folds) and presents with greasy scales, not photosensitivity. * **Contact Dermatitis:** This is a Type IV hypersensitivity reaction to an external allergen (e.g., nickel, cement). While it can occur on exposed skin, it is not triggered by porphyria or specifically seasonal/sun-dependent unless it is "photoallergic," which doesn't fit the patient's history. * **Psoriasis:** Characterized by well-demarcated erythematous plaques with silvery scales. Interestingly, sunlight usually *improves* psoriasis rather than exacerbating it. **High-Yield Clinical Pearls for NEET-PG:** * **Key Association:** PCT is strongly associated with **Hepatitis C virus (HCV)** infection, alcohol consumption, and iron overload. * **Diagnosis:** Look for "tea-colored" or "port-wine" urine. Wood’s lamp examination of urine shows **coral-red fluorescence**. * **Treatment:** Small-volume phlebotomy (to reduce iron) or low-dose hydroxychloroquine. * **Hallmark:** Hypertrichosis (increased hair growth) on the temples and malar area is a common associated finding in PCT.
Question 10: All of the following are true about solar urticaria, EXCEPT:
- A. Common in females between age group above 40 years (Correct Answer)
- B. Lesions subside spontaneously on avoiding exposure within 24 hours
- C. Some cases may develop severe urticaria/bronchospasm
- D. Almost all cases are idiopathic
Explanation: **Solar Urticaria** is a rare form of physical urticaria characterized by the rapid onset of wheals and erythema within minutes of exposure to ultraviolet (UV) or visible light. ### **Explanation of Options:** * **Option A (Correct Answer):** This statement is incorrect because solar urticaria typically affects **young adults**, with a peak onset between **20 and 40 years** of age. While it does show a slight female preponderance, it is not primarily a disease of the elderly or those above 40. * **Option B:** This is a classic feature. The wheals appear within 5–10 minutes of sun exposure and characteristically **resolve within 1–24 hours** (usually within 1–2 hours) once the patient moves into the shade. * **Option C:** In cases of extensive body surface area exposure, patients can experience systemic symptoms due to massive histamine release, including **bronchospasm, hypotension, and syncope** (anaphylactoid reaction). * **Option D:** Most cases are **idiopathic**. It is hypothesized to be a Type I hypersensitivity reaction where an endogenous precursor (chromophore) is converted into an allergen by specific wavelengths of light. ### **High-Yield Clinical Pearls for NEET-PG:** 1. **Action Spectrum:** Most commonly triggered by **UVA** (320–400 nm) and **Visible Light**, though UVB can also be a trigger. 2. **Diagnosis:** Confirmed by **Phototesting**, where the skin is exposed to specific wavelengths to determine the Minimal Urticarial Dose (MUD). 3. **The "Fixed" Variant:** Unlike most urticarias, some patients have a "fixed" solar urticaria where lesions only appear on specific skin sites. 4. **Management:** First-line treatment involves **H1-antihistamines**. For refractory cases, **Photodesensitization** (hardening) or Omalizumab may be used. 5. **Differential Diagnosis:** Must be distinguished from Polymorphous Light Eruption (PMLE), which takes hours to days to appear and days to resolve.
Practice by Chapter
Fundamentals of Photobiology
Practice Questions
UVA and UVB Phototherapy
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PUVA Therapy
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Narrow-Band UVB Therapy
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Excimer Laser Therapy
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Photodynamic Therapy
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Phototoxicity and Photoallergy
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Photoprotection
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Sunscreens
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Photoaging
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Phototherapy Protocols
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Management of Phototherapy Side Effects
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