Phototherapy and Photobiology — MCQs

Phototherapy and Photobiology Indian Medical PG Practice Questions and MCQs

Question 11: What is the mechanism of action of psoralen?

A. Binding to DNA (Correct Answer)
B. Inhibiting protein synthesis
C. Inhibiting angiogenesis
D. Inhibiting keratinization

Explanation: **Explanation:** The correct answer is **A. Binding to DNA**. **Mechanism of Action:** Psoralens (e.g., 8-Methoxypsoralen) are photosensitizing agents used in **PUVA (Psoralen + UVA)** therapy. The mechanism occurs in two distinct phases: 1. **Type I Reaction (Oxygen-independent):** Psoralens intercalate between the base pairs of the DNA helix. Upon exposure to UVA radiation (320–400 nm), they form **monoadducts** and **bifunctional adducts (interstrand cross-links)** with pyrimidine bases (primarily thymine). This inhibits DNA synthesis and keratinocyte proliferation, which is the primary therapeutic effect in psoriasis. 2. **Type II Reaction (Oxygen-dependent):** UVA excites the psoralen molecule, transferring energy to molecular oxygen to create reactive oxygen species (ROS), leading to cell membrane damage and melanocyte stimulation. **Why other options are incorrect:** * **B & D:** While PUVA ultimately leads to decreased protein synthesis and altered keratinization as a *downstream result* of DNA damage and cell cycle arrest, these are not the primary molecular mechanisms. * **C:** Inhibiting angiogenesis is a feature of certain systemic retinoids or biologicals, but it is not the defining mechanism of psoralens. **High-Yield Clinical Pearls for NEET-PG:** * **Action Spectrum:** UVA (320–400 nm) is required to activate psoralen. * **Indications:** Psoriasis, Vitiligo (stimulates melanocytes), and Mycosis Fungoides (CTCL). * **Side Effects:** Acute (erythema, pruritus) and Chronic (Photoaging, **Squamous Cell Carcinoma**). * **Ocular Safety:** Patients must wear UVA-blocking goggles for 24 hours after ingestion to prevent **cataracts**, as psoralen distributes to the lens.

Question 12: A 45-year-old farmer presents with itchy erythematous papular lesions on the face, neck, 'V' area of the chest, and dorsum of hands and forearms, present for 3 years. The lesions are more severe in summers and improve in winters. What is the most appropriate diagnostic test for this condition?

A. Skin biopsy (Correct Answer)
B. Estimation of IgE levels in blood
C. Patch test
D. Intradermal prick test

Explanation: ### Explanation **Diagnosis: Chronic Actinic Dermatitis (CAD)** The clinical presentation—a middle-aged farmer with itchy, erythematous lesions in a **photo-distributed pattern** (face, V-area of chest, dorsum of hands) that exacerbates in summer—is classic for **Chronic Actinic Dermatitis (CAD)**. CAD is an immunologically mediated photodermatosis that often begins as an eczematous reaction to light. **1. Why Skin Biopsy is the Correct Answer:** While the diagnosis of CAD is primarily clinical, a **skin biopsy** is the most appropriate diagnostic step to confirm the diagnosis and, more importantly, to **rule out mimics** like Mycosis Fungoides (Cutaneous T-cell Lymphoma). Histopathology in CAD typically shows an eczematous pattern (spongiosis) or, in severe cases (Actinic Reticuloid), a dense dermal infiltrate that can resemble lymphoma. **2. Why Other Options are Incorrect:** * **Patch Test (Option C):** While many CAD patients have associated contact allergies (e.g., to Compositae plants or fragrances), a patch test identifies contact allergens, not the primary photosensitivity. A *Photo-patch test* would be more relevant, but biopsy remains the gold standard for definitive characterization. * **Estimation of IgE (Option B):** Elevated IgE is a marker for Atopic Dermatitis, not photodermatoses. * **Intradermal Prick Test (Option D):** This is used to diagnose Type I hypersensitivity (e.g., allergic rhinitis or asthma) and has no role in diagnosing CAD. **Clinical Pearls for NEET-PG:** * **CAD Spectrum:** Includes Photosensitive Eczema and Actinic Reticuloid. * **Demographics:** Typically affects elderly males (often outdoor workers like farmers). * **Photo-testing:** Patients show a **reduced Minimal Erythema Dose (MED)** to UVA, UVB, and sometimes visible light. * **Management:** Strict photoprotection, topical steroids, and in refractory cases, systemic immunosuppressants (Azathioprine).

Question 13: What is the typical wavelength range of a Wood's lamp?

A. 360-385 nm (Correct Answer)
B. 280-320 nm
C. 400-450 nm
D. 450-500 nm

Explanation: ### Explanation **1. Why Option A is Correct:** A Wood’s lamp (also known as a black light) is a diagnostic tool that emits long-wave Ultraviolet A (UVA) radiation. The device uses a high-pressure mercury arc lamp with a specialized filter called a **Wood’s filter** (composed of barium silicate with 9% nickel oxide). This filter is opaque to all light except for a specific band in the UVA spectrum, typically peaking at **365 nm**, with a functional range of **360–385 nm**. When this light hits certain substances (fluorophores) on the skin or hair, they absorb the energy and re-emit it as visible light (fluorescence). **2. Why Other Options are Incorrect:** * **Option B (280–320 nm):** This represents the **UVB spectrum**. This range is used in phototherapy (e.g., Narrowband UVB at 311 nm) for psoriasis and vitiligo but is not the wavelength of a Wood’s lamp. * **Option C (400–450 nm):** This is the **Visible Blue Light** range. While used in treating neonatal jaundice or PDT, it is beyond the UV spectrum of a Wood’s lamp. * **Option D (450–500 nm):** This is the **Cyan/Green visible light** range, which does not possess the properties required for diagnostic fluorescence in dermatology. **3. Clinical Pearls for NEET-PG:** * **Tinea Capitis:** *Microsporum* species show **Brilliant Green** fluorescence (due to Pteridine). *Trichophyton schoenleinii* shows **Dull Blue**. * **Erythrasma:** Caused by *Corynebacterium minutissimum*; shows **Coral Red** fluorescence (due to Coproporphyrin III). * **Porphyria Cutanea Tarda:** Urine shows **Pink-Orange** fluorescence. * **Pseudomonas:** Shows **Yellow-Green** fluorescence (due to Pyoverdin). * **Vitiligo:** Shows **Milky White** fluorescence (helps distinguish from pityriasis alba, which does not fluoresce). * **Note:** Always perform the examination in a **dark room** to visualize the weak fluorescence.

Question 14: All are true regarding Polymorphous light eruption (PMLE), EXCEPT?

A. Rash caused by sun exposure
B. Rash triggered by exposure to artificial ultraviolet (UV) light
C. Healing without scarring
D. Face is always affected (Correct Answer)

Explanation: **Explanation:** Polymorphous Light Eruption (PMLE) is the most common idiopathic primary photodermatosis. It is a delayed-type hypersensitivity reaction to an endogenous antigen induced by UV radiation. **Why Option D is the Correct Answer (The Exception):** The **face is frequently spared** in PMLE. This is due to the phenomenon of **"hardening"** (photo-tolerance). Areas chronically exposed to the sun, like the face and back of the hands, develop a tolerance to UV light. PMLE characteristically affects "intermittently exposed" areas, such as the V-area of the neck, forearms, and upper arms, especially during the first strong sun exposure of spring or summer. **Analysis of Other Options:** * **Option A & B:** PMLE is triggered by sunlight, specifically **UVA** (most common) and sometimes UVB or even artificial UV sources (tanning beds). The rash typically appears hours to days after exposure. * **Option C:** Despite the "polymorphic" nature of the lesions (papules, vesicles, or plaques), they are superficial and inflammatory. They resolve spontaneously within 7–10 days if further sun exposure is avoided, **healing without any permanent scarring.** **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Most common presentation is the **papular variant**. * **Seasonality:** Typically occurs in spring/early summer; improves as summer progresses due to the "hardening" effect. * **Gender Predilection:** More common in females (2:1 ratio). * **Management:** Photoprotection (broad-spectrum sunscreens) is first-line. Severe cases may require topical steroids or "prophylactic desensitization" using narrow-band UVB (NB-UVB) or PUVA.

Question 15: On a 4 Gy dose of radiation to the scalp, when does alopecia typically develop?

A. 3-4 weeks (Correct Answer)
B. 6-8 weeks
C. 10-12 weeks
D. 14-16 weeks

Explanation: **Explanation:** The correct answer is **A. 3-4 weeks.** **Medical Concept:** Radiation-induced alopecia occurs due to the high mitotic activity of hair follicle matrix cells, which makes them exquisitely sensitive to ionizing radiation. A dose of **4 Gy (400 rads)** is sufficient to cause **temporary effluvium**. Following exposure, the hair follicles enter a dystrophic state, leading to hair shedding. This process typically manifests clinically after a latent period of **3 to 4 weeks**, as it takes time for the damaged hair shaft to separate and be extruded from the follicle. **Analysis of Options:** * **A (3-4 weeks):** Correct. This matches the physiological timeline of hair matrix arrest and subsequent shedding following a single sub-epilating or epilating dose of radiation. * **B, C, and D:** These timelines are too late. While permanent alopecia (occurring at doses >7 Gy) results in long-term hair loss, the initial shedding still begins within the first month. Waiting 6 to 16 weeks would be more characteristic of *telogen effluvium* (triggered by systemic stress) rather than acute radiation injury. **High-Yield Clinical Pearls for NEET-PG:** * **Threshold Doses:** * **3-5 Gy:** Causes temporary alopecia (regrowth begins in 2-4 months). * **>7-10 Gy:** Can cause permanent alopecia due to irreversible damage to the stem cells in the hair bulge. * **Radiotherapy & Hair:** The most common type of hair loss post-radiation is **Anagen Effluvium**, where the hair shaft narrows (Pohl-Pinkus marks) and breaks. * **Historical Context:** In the past, the "Adamson-Kienbock point technique" used X-rays to induce epilation for treating Tinea Capitis, aiming for this exact 3-week window.

Question 16: Which of the following skin conditions is treated with PUVA therapy?

A. Psoriasis (Correct Answer)
B. Tinea versicolor
C. Lichen planus
D. Porphyria cutanea tarda

Explanation: **Explanation** **PUVA (Psoralen + Ultraviolet A)** therapy is a form of photochemotherapy that combines the administration of a photosensitizing agent (Psoralen) with exposure to UVA radiation (320–400 nm). **1. Why Psoriasis is correct:** Psoriasis is a chronic inflammatory skin disorder characterized by keratinocyte hyperproliferation and T-cell mediated inflammation. PUVA works by intercalating Psoralen into the DNA of rapidly dividing cells. Upon exposure to UVA, it forms DNA cross-links (cyclobutane dimers), which inhibit DNA synthesis and suppress cellular proliferation. Additionally, PUVA induces apoptosis of T-lymphocytes, leading to local immunosuppression and clearing of psoriatic plaques. **2. Why other options are incorrect:** * **Tinea versicolor:** This is a superficial fungal infection caused by *Malassezia furfur*. It is treated with topical or systemic antifungals (e.g., Ketoconazole), not phototherapy. * **Lichen planus:** While Narrowband UVB (NBUVB) can be used for generalized lichen planus, PUVA is not the primary treatment. Furthermore, in the context of standard NEET-PG questions, Psoriasis remains the "gold standard" indication for PUVA. * **Porphyria cutanea tarda (PCT):** This is a metabolic disorder of heme synthesis. UV light actually *exacerbates* porphyrias (photosensitivity). Treatment involves phlebotomy or low-dose hydroxychloroquine. **Clinical Pearls for NEET-PG:** * **Wavelength:** UVA (320–400 nm); NBUVB (311–312 nm). NBUVB is now preferred over PUVA due to a lower risk of skin cancer. * **Side Effects:** Acute (erythema, pruritus, nausea) and Chronic (Photoaging, **Squamous Cell Carcinoma**, and "PUVA lentigines"). * **Other Indications:** Vitiligo, Mycosis Fungoides (CTCL), and Alopecia Areata. * **Contraindication:** Xeroderma pigmentosum, Systemic Lupus Erythematosus (SLE), and pregnancy.

Question 17: Solar urticaria is characterized by which of the following skin reactions after exposure to the sun?

A. Stinging
B. Erythema
C. Wheal formation (Correct Answer)
D. Loss of sensation

Explanation: **Explanation:** **Solar Urticaria** is a rare form of physical urticaria triggered by exposure to ultraviolet (UV) or visible light. 1. **Why Wheal formation is correct:** The underlying pathophysiology involves a **Type I (IgE-mediated) hypersensitivity reaction**. When the skin is exposed to specific wavelengths of light, an endogenous chromophore (photoallergen) is activated. This leads to the degranulation of mast cells and the release of histamine and other inflammatory mediators. Clinically, this manifests as the rapid onset (within minutes) of **wheals** (hives), itching, and swelling, which typically resolve within 24 hours once the light source is removed. 2. **Why other options are incorrect:** * **Stinging:** While itching or burning may occur, stinging is more characteristic of *Polymorphous Light Eruption (PMLE)* or chemical photosensitivity. * **Erythema:** Although erythema (redness) often surrounds the wheal, it is a non-specific finding. The hallmark diagnostic feature of urticaria is the transient, palpable wheal. * **Loss of sensation:** This is not a feature of photodermatoses; it would suggest a neurological deficit or conditions like Leprosy. **High-Yield Clinical Pearls for NEET-PG:** * **Action Spectrum:** Most commonly triggered by **UVA** and visible light. * **Diagnosis:** Confirmed via **Phototesting**, where the skin is exposed to specific wavelengths to induce a wheal. * **Management:** First-line treatment includes **H1-antihistamines**. For refractory cases, "hardening" (photodesensitization) or Omalizumab may be used. * **Differential:** Unlike PMLE (which takes hours/days to appear), Solar Urticaria appears within **5–10 minutes** of exposure.

Question 18: A 32-year-old male presents with abdominal pain and vomiting. He also complains of psychiatric symptoms and visual hallucinations. What is the most likely diagnosis?

A. Hypothyroidism
B. Hyperthyroidism
C. Hysteria
D. Intermittent Porphyria (Correct Answer)

Explanation: ### Explanation The correct diagnosis is **Acute Intermittent Porphyria (AIP)**. This condition is an autosomal dominant metabolic disorder caused by a deficiency in the enzyme **Porphobilinogen deaminase (PBG-D)**, leading to the accumulation of porphyrin precursors like ALA and PBG. **Why it is correct:** AIP is classically characterized by the **"Triad of Ps"**: 1. **P**ain in abdomen (most common symptom, often severe and colicky). 2. **P**sychological symptoms (anxiety, agitation, hallucinations, or psychosis). 3. **P**olyneuropathy (motor weakness). The patient’s presentation of abdominal pain, vomiting, and visual hallucinations fits the classic "neurovisceral" attack of AIP. Notably, unlike other porphyrias, AIP **does not** present with cutaneous photosensitivity. **Why other options are incorrect:** * **Hypo/Hyperthyroidism:** While thyroid disorders can cause psychiatric symptoms (depression in hypo; anxiety/psychosis in hyper), they do not typically present with acute, severe abdominal pain and vomiting as the primary clinical feature. * **Hysteria:** This is a diagnosis of exclusion. Given the systemic nature of the symptoms (vomiting and abdominal pain), an organic cause like porphyria must be ruled out first. AIP is often misdiagnosed as a psychiatric disorder or a surgical emergency. **NEET-PG High-Yield Pearls:** * **Enzyme Defect:** Porphobilinogen deaminase (also known as Hydroxymethylbilane synthase). * **Urine Finding:** Urine turns **"Port-wine red"** on standing due to the oxidation of porphobilinogen to porphobilin. * **Precipitating Factors:** Barbiturates, sulfonamides, alcohol, and fasting (the "5 Ps": **P**ills, **P**upping/Pregnancy, **P**aint/Alcohol, **P**hage/Starvation, **P**eriods). * **Treatment:** Intravenous **Hemin** (suppresses ALA synthase) and high-dose glucose.

Question 19: A 26-year-old female with a history of extensive exposure to sun comes to your clinic with presentation shown below. What is the most likely diagnosis?

A. Urticaria
B. Dermatitis herpetiformis
C. Erythema marginatum
D. Dermatomyositis (Correct Answer)

Explanation: ***Dermatomyositis*** - The image shows **Gottron's papules** (violaceous, flat-topped papules over bony prominences like knuckles) or a **heliotrope rash** (periorbital violaceous rash), which are classic skin findings in dermatomyositis, an inflammatory myopathy. - The history of **sun exposure** is relevant as dermatomyositis rashes are often **photosensitive**, and the patient's age and gender are consistent with typical presentation. *Urticaria* - Urticaria presents as rapidly appearing and disappearing **wheals** (hives) that are typically itchy and erythematous with central pallor. - The lesions in the image appear more fixed and papular rather than evanescent wheals, making urticaria less likely. *Dermatitis herpetiformis* - Dermatitis herpetiformis is characterized by intensely **pruritic vesicles and bullae** symmetrically distributed on extensor surfaces, often associated with celiac disease. - The appearance in the image does not show blistering or the characteristic distribution of dermatitis herpetiformis. *Erythema marginatum* - Erythema marginatum is a rare, transient, and non-pruritic serpiginous (snake-like) rash with sharply demarcated borders and central clearing, typically seen in **acute rheumatic fever**. - The rash in the image lacks the characteristic migratory, annular, or serpiginous pattern of erythema marginatum.

Question 20: What is the best time to give oral psoralen with UVA -

A. Half an hour after UVA
B. 1 hour before UVA (Correct Answer)
C. 1 hour after UVA
D. Half hour before UVA

Explanation: ***1 hour before UVA*** - Oral psoralen, when used in **PUVA therapy**, requires adequate time for absorption and distribution to the skin to exert its photosensitizing effects. - Administering psoralen approximately **1-2 hours before UVA exposure** (typically 1.5-2 hours for 8-methoxypsoralen) ensures peak drug concentration in the skin, maximizing therapeutic efficacy. - Among the given options, **1 hour is the most appropriate timing** as it allows sufficient absorption while being closer to the therapeutic window than shorter intervals. *Half an hour after UVA* - Administering psoralen **after UVA exposure** would mean the photosensitizer is not present in sufficient concentrations during the light therapy. - This timing would render the UVA treatment largely ineffective as the primary mechanism of action, **photoreaction with UVA**, would not occur. *1 hour after UVA* - Giving psoralen **1 hour after UVA** completely misses the therapeutic window needed for the drug to photosensitize the skin during light exposure. - The UVA light would have already been administered without the active drug in the skin, thus providing **minimal to no therapeutic benefit**. *Half hour before UVA* - **Half an hour before UVA** is insufficient time for adequate oral absorption and distribution of psoralen to achieve optimal photosensitization. - This could result in **suboptimal drug concentration at the time of UVA exposure** and reduced therapeutic efficacy compared to the standard 1-2 hour pre-treatment timing.

Practice by Chapter

Fundamentals of Photobiology

Practice Questions

UVA and UVB Phototherapy

Practice Questions

PUVA Therapy

Practice Questions

Narrow-Band UVB Therapy

Practice Questions

Excimer Laser Therapy

Practice Questions

Photodynamic Therapy

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Phototoxicity and Photoallergy

Practice Questions

Photoprotection

Practice Questions

Sunscreens

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Photoaging

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Phototherapy Protocols

Practice Questions

Management of Phototherapy Side Effects

Practice Questions

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