Cytopathology — MCQs

A 75-year-old man with no past medical history presents with increasing shortness of breath over 6 months. He previously worked at a shipyard with significant exposure to asbestos. He has a large right-sided pleural effusion. Pleural aspiration reveals an exudate, but cytopathological examination identifies no malignant cells. CT scanning reveals circumferential pleural thickening but no other abnormalities. Which test is most likely to yield a diagnosis?

Q36Medium

A 4-year-old boy with a history of intellectual disability and seizures presents with a one-month history of shortness of breath. Physical examination reveals numerous papules on the face. Echocardiography shows tumor-like growth in the left ventricle. What is the most likely diagnosis for this patient's condition?

Q37Easy

What fixative is used for bone histopathology?

Q38Easy

What are the histopathological findings in cysticercosis?

Q39Medium

A 25-year-old man presents with right knee pain, described as "popping and catching," for the past 2 years. Examination reveals a reduced range of motion in the right knee without tenderness or swelling. MRI shows a 3-cm mass in the anterior joint space. Arthroscopic partial synovectomy is performed, and a nodular, encapsulated mass is removed. Microscopic examination of the mass reveals synoviocyte-like tumor cells in a hyalinized stroma containing osteoclast-like giant cells and hemosiderin. An antagonist to which of the following growth factors is most useful in treating this lesion?

Q40Easy

Which stain is used for sebaceous cell carcinoma?

Cytopathology Indian Medical PG Practice Questions and MCQs

Question 31: Which of the following is the correct fixative agent for a Pap smear?

A. Normal saline
B. 95% ethanol (Correct Answer)
C. Formalin
D. Air drying

Explanation: **Explanation:** In cytopathology, the primary goal of fixation is to preserve cellular morphology, nuclear detail, and chromatin structure. For a **Papanicolaou (Pap) smear**, the gold standard fixative is **95% Ethyl alcohol (Ethanol)** [1]. 1. **Why 95% Ethanol is Correct:** Ethanol is a **dehydrating (coagulant) fixative**. It works by removing water and causing the proteins to denature and precipitate in situ. This process hardens the cell and "locks" the nuclear chromatin in place, which is essential for identifying the nuclear features of malignancy (e.g., hyperchromasia, pleomorphism) [1]. 2. **Why other options are incorrect:** * **Normal Saline:** This is a transport medium, not a fixative. Leaving cells in saline for too long leads to cellular swelling and autolysis. * **Formalin (10% Neutral Buffered Formalin):** This is the standard fixative for **histopathology** (tissue biopsies), not cytopathology. Formalin creates cross-links that can make the cell membrane impermeable to the specific dyes used in the Pap stain. * **Air drying:** While used for Romanowsky stains (like Leishman or MGG) to study cytoplasmic detail or blood films, air drying causes **"drying artifacts"** in Pap smears. This results in nuclear swelling and loss of crispness, making it impossible to accurately grade cervical dysplasia [2]. **High-Yield Clinical Pearls for NEET-PG:** * **Ideal Fixation Time:** Minimum 15–30 minutes. * **Alternative Fixatives:** If 95% ethanol is unavailable, **100% methanol** or **95% isopropyl alcohol** can be used. * **Carnoy’s Fluid:** Used specifically for **bloody smears** as it lyses Red Blood Cells (RBCs) while fixing the cells. * **Spray Fixatives:** Often contain polyethylene glycol (PEG) which forms a protective thin film over the cells, allowing for easy transport. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Female Genital Tract, pp. 1010-1011. [2] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Female Genital Tract Disease, pp. 468-470.

Question 32: A 66-year-old man presents with urinary frequency, hesitancy, and dysuria. Digital rectal examination shows an enlarged, rubbery, and nodular prostate. Serum PSA is modestly elevated. Which of the following is most closely related to the pathogenesis of this patient's likely condition?

A. Dihydrotestosterone (DHT) (Correct Answer)
B. Estrogen
C. Alpha-fetoprotein (AFP)
D. Testosterone

Explanation: **Explanation:** The clinical presentation of urinary frequency, hesitancy, and a rubbery, nodular prostate on digital rectal examination (DRE) in an elderly male is classic for **Benign Prostatic Hyperplasia (BPH)**. **Why Dihydrotestosterone (DHT) is correct:** DHT is the primary mediator of prostatic growth [1]. In the prostate, the enzyme **5-alpha reductase (Type 2)** converts circulating testosterone into DHT [2]. DHT has a much higher affinity for androgen receptors than testosterone. Once bound, it stimulates the transcription of growth factors (like FGF and TGF-̢) that lead to the proliferation of stromal and epithelial cells, primarily in the **transition zone** of the prostate [1], [4]. **Why other options are incorrect:** * **Testosterone:** While it is the precursor, it is not the active mediator within the prostate. Patients with a genetic deficiency of 5-alpha reductase do not develop BPH, proving that DHT, not testosterone, is the culprit. * **Estrogen:** While estrogen levels increase with age and may increase the expression of androgen receptors, it is a secondary factor rather than the primary driver of pathogenesis. * **Alpha-fetoprotein (AFP):** This is a tumor marker for yolk sac tumors and hepatocellular carcinoma; it has no role in prostatic pathology. **High-Yield NEET-PG Pearls:** * **Location:** BPH occurs in the **Transition Zone** (leads to early urethral compression), whereas Prostate Cancer typically occurs in the **Peripheral Zone** [1], [4]. * **Microscopy:** Characterized by hyperplasia of both glandular and stromal elements [3]. Look for **Corpora Amylacea** (pink hyaline lamellated bodies) in the glandular lumina. * **Pharmacology Link:** 5-alpha reductase inhibitors (e.g., **Finasteride**) are used to shrink the prostate by decreasing DHT levels [2]. * **PSA:** Modest elevations (4–10 ng/mL) are common in BPH; levels >10 ng/mL are highly suspicious for malignancy [2]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Lower Urinary Tract and Male Genital System, pp. 986-988. [2] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Diseases Of The Urinary And Male Genital Tracts, pp. 499-500. [3] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Diseases Of The Urinary And Male Genital Tracts, pp. 498-499. [4] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Diseases Of The Urinary And Male Genital Tracts, pp. 496-497.

Question 33: Which of the following are histological features of medulloblastoma?

A. Small round blue cells
B. Scant amount of cytoplasm and hyperchromatic nuclei
C. Homer-Wright rosettes
D. All of the above (Correct Answer)

Explanation: **Medulloblastoma** is a highly malignant, Grade 4 embryonal tumor of the cerebellum, most commonly occurring in the pediatric population. It belongs to the family of **Small Round Blue Cell Tumors (SRBCTs)**, which dictates its classic histological appearance. [2] ### **Explanation of Features:** * **Small Round Blue Cells (Option A):** The tumor is composed of densely packed, primitive cells. Because these cells have very little cytoplasm and large, dark nuclei, the tissue appears intensely blue (basophilic) on H&E staining. * **Scant Cytoplasm & Hyperchromatic Nuclei (Option B):** These are hallmark features of "primitive" or "embryonal" cells. The nuclei are often elongated or carrot-shaped, showing significant pleomorphism and frequent mitotic figures. [3] * **Homer-Wright Rosettes (Option C):** This is a classic diagnostic feature seen in approximately 40% of cases. These are "pseudorosettes" where tumor cells are arranged in a circle around a central fibrillar core (neuropil), without a central lumen or blood vessel. [1] Since all three descriptions are characteristic histological findings of medulloblastoma, **Option D (All of the above)** is the correct answer. ### **High-Yield Clinical Pearls for NEET-PG:** * **Location:** Most commonly arises in the **vermis** of the cerebellum in children (protruding into the 4th ventricle) and the cerebellar hemispheres in adults. * **Molecular Subtypes:** WNT (best prognosis), SHH, Group 3 (worst prognosis), and Group 4. [2] * **Spread:** Characterized by "drop metastasis" via CSF seeding to the cauda equina. [2] * **Differential Diagnosis:** Must be distinguished from Ependymoma, which features **Perivascular Pseudorosettes** (cells around a blood vessel) and **True Ependymal Rosettes** (cells around a central lumen). [4] **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of Infancy and Childhood, pp. 484-485. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Central Nervous System, pp. 1314-1315. [3] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Manifestations Of Central And Peripheral Nervous System Disease, pp. 726-727. [4] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Central Nervous System, pp. 1312-1313.

Question 34: Post-streptococcal glomerulonephritis in children is diagnosed by?

A. Heavy proteinuria, high cholesterol, high ASO titer
B. Heavy proteinuria, hematuria, low ASO titer
C. Mild proteinuria, hematuria, high ASO titer (Correct Answer)
D. Mild proteinuria, high cholesterol, normal ASO titer

Explanation: **Explanation:** Post-streptococcal glomerulonephritis (PSGN) is a classic example of **Nephritic Syndrome**, occurring typically 1–3 weeks after a Group A Beta-hemolytic Streptococcal infection (pharyngitis or impetigo) [1]. **1. Why Option C is Correct:** * **Hematuria:** This is the hallmark of nephritic syndrome, often presenting as "cola-colored" or smoky urine due to glomerular capillary damage. [1] * **Mild Proteinuria:** Unlike nephrotic syndrome, proteinuria in PSGN is usually sub-nephrotic (<3.5 g/day), often described as "mild to moderate." * **High ASO Titer:** Evidence of a recent streptococcal infection is essential for diagnosis. Anti-streptolysin O (ASO) titers are elevated in 90% of cases following pharyngitis (Anti-DNAse B is more sensitive for skin infections). **2. Why Other Options are Incorrect:** * **Options A & B (Heavy Proteinuria):** Heavy (nephrotic-range) proteinuria and high cholesterol are characteristic of **Nephrotic Syndrome** (e.g., Minimal Change Disease), not PSGN. [1] * **Options B & D (Low/Normal ASO):** PSGN is an immune-mediated sequela of a specific bacterial infection; therefore, streptococcal antibodies (ASO, Anti-DNAse B, or Streptozyme) must be elevated to confirm the etiology. [1] **3. High-Yield Clinical Pearls for NEET-PG:** * **Complement Levels:** Characterized by **low C3** levels (due to consumption via the alternative pathway). C3 levels typically return to normal within 6–8 weeks. * **Light Microscopy:** Shows "Starry sky" appearance or diffuse proliferative glomerulonephritis with enlarged, hypercellular glomeruli. [1] * **Electron Microscopy:** Pathognomonic **"Sub-epithelial humps"** (immune complex deposits). [1] * **Immunofluorescence:** "Lumpy-bumpy" or granular deposits of IgG and C3. [1] **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Kidney, pp. 914-916.

Question 35: A 75-year-old man with no past medical history presents with increasing shortness of breath over 6 months. He previously worked at a shipyard with significant exposure to asbestos. He has a large right-sided pleural effusion. Pleural aspiration reveals an exudate, but cytopathological examination identifies no malignant cells. CT scanning reveals circumferential pleural thickening but no other abnormalities. Which test is most likely to yield a diagnosis?

A. Thoracoscopy (Correct Answer)
B. Bronchoscopy
C. Echocardiogram
D. Repeat pleural aspiration

Explanation: ### Explanation **Correct Option: A. Thoracoscopy** The clinical presentation—an elderly patient with significant **asbestos exposure**, chronic shortness of breath, and **circumferential pleural thickening**—is highly suspicious for **Malignant Mesothelioma** [4]. 1. **Why Thoracoscopy?** Pleural fluid cytology has a notoriously low sensitivity (approx. 30–50%) for diagnosing mesothelioma because malignant mesothelial cells are often difficult to distinguish from reactive mesothelial cells. Thoracoscopy (VATS) is the gold standard because it allows for direct visualization of the pleura and the collection of **large, multiple, targeted biopsies**. This provides the tissue architecture (showing stromal invasion) necessary for a definitive histopathological and immunohistochemical diagnosis. **Analysis of Incorrect Options:** * **B. Bronchoscopy:** This is used for endobronchial lesions or central lung tumors. Mesothelioma is a pleural disease and does not typically involve the bronchial tree. * **C. Echocardiogram:** While it can rule out heart failure as a cause of effusion, the CT findings of circumferential pleural thickening and the history of asbestos exposure point toward a primary pleural malignancy [2]. * **D. Repeat pleural aspiration:** Since the initial cytology was negative, repeating the procedure is unlikely to increase the yield significantly. Mesothelioma requires tissue architecture for diagnosis, which aspiration cannot provide. ### High-Yield Clinical Pearls for NEET-PG: * **Asbestos Exposure:** The most common cancer associated with asbestos is **Bronchogenic Carcinoma**, but the most specific cancer is **Mesothelioma** [3]. * **Latency Period:** Mesothelioma has a long latency period (often 25–40 years after exposure) [4]. * **Cytopathology:** Look for "windows" between cells and "knobby" clusters in mesothelioma cytology. * **IHC Markers:** Mesothelioma is typically **Calretinin (+)**, WT-1 (+), and Cytokeratin 5/6 (+), while it is **CEA (-)** [1]. * **Radiology:** "Pleural rind" or circumferential thickening is a classic sign of malignancy. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Lung, p. 731. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Lung, pp. 728-729. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Lung, p. 699. [4] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Respiratory Tract Disease, pp. 339-340.

Question 36: A 4-year-old boy with a history of intellectual disability and seizures presents with a one-month history of shortness of breath. Physical examination reveals numerous papules on the face. Echocardiography shows tumor-like growth in the left ventricle. What is the most likely diagnosis for this patient's condition?

A. Lipoma
B. Myxoma
C. Rhabdomyoma (Correct Answer)
D. Dilated cardiomyopathy

Explanation: ### Explanation The clinical presentation is a classic description of **Tuberous Sclerosis Complex (TSC)**, a multi-system genetic disorder [1]. The combination of **intellectual disability, seizures**, and facial papules (**angiofibromas**) constitutes the classic Vogt’s triad [1]. In pediatric patients with TSC, the most common primary cardiac tumor is a **Rhabdomyoma** [1], [4]. **Why Rhabdomyoma is correct:** * **Association:** Up to 80% of patients with cardiac rhabdomyomas have Tuberous Sclerosis [1]. * **Clinical Features:** These are benign hamartomas of cardiac muscle [1]. While often asymptomatic, they can cause outflow tract obstruction (leading to shortness of breath) or arrhythmias. * **Cytopathology/Histology:** High-yield for exams is the presence of **"Spider cells"**—large cells with central nuclei and thin cytoplasmic strands extending to the periphery, resembling a spider web. **Why other options are incorrect:** * **Lipoma:** While these can occur in the heart, they are rare and not associated with the neurocutaneous markers (seizures, facial papules) seen in this case [4]. * **Myxoma:** This is the most common primary cardiac tumor in **adults**, typically located in the **left atrium** [2], [4]. It is not associated with Tuberous Sclerosis. * **Dilated cardiomyopathy:** This presents with global ventricular enlargement and systolic dysfunction, not a focal "tumor-like growth" [3]. **NEET-PG High-Yield Pearls:** * **Most common cardiac tumor in children:** Rhabdomyoma. * **Most common cardiac tumor in adults:** Myxoma [4]. * **Most common tumor of the heart (overall):** Metastatic tumors (e.g., from lung, breast, or melanoma). * **Tuberous Sclerosis Markers:** Ash-leaf spots (hypopigmented macules), Shagreen patches, and Renal Angiomyolipomas [1]. * **Natural History:** Cardiac rhabdomyomas in TSC often undergo spontaneous regression during early childhood. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Central Nervous System, pp. 1318-1319. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Heart, pp. 583-584. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Heart, p. 576. [4] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Cardiovascular Disease, pp. 304-306.

Question 37: What fixative is used for bone histopathology?

A. 10% formalin (Correct Answer)
B. Normal saline
C. Rectified spirit
D. Nothing

Explanation: **Explanation:** The primary goal of fixation in histopathology is to preserve the tissue in as life-like a state as possible by preventing autolysis and putrefaction. **1. Why 10% Formalin is Correct:** **10% Neutral Buffered Formalin (NBF)** is the standard universal fixative for almost all surgical pathology specimens, including bone. It works by creating additive cross-links between proteins (methylene bridges). For bone specimens, fixation in formalin is a mandatory first step before the tissue can undergo **decalcification** (the removal of calcium ions using acids like EDTA or Nitric acid). Without proper formalin fixation, the acidic decalcifying agents would macerate and destroy the cellular morphology of the bone marrow and osteocytes [2]. **2. Why the Other Options are Incorrect:** * **Normal Saline (B):** This is an isotonic solution, not a fixative. It is used only for temporary transport (e.g., for fresh frozen sections or immunofluorescence) but leads to rapid autolysis if the tissue is left in it. * **Rectified Spirit (C):** While alcohols are fixatives, they cause significant tissue shrinkage and hardening. They are primarily used in cytopathology (e.g., Pap smears) rather than bulk tissue histopathology. * **Nothing (D):** Leaving tissue "dry" leads to rapid desiccation and cell death, rendering the sample non-diagnostic. **Clinical Pearls for NEET-PG:** * **Gold Standard:** 10% Formalin is the most common answer for "best fixative" unless a specific tissue like the eye (Davidson’s) or testis (Bouin’s) is mentioned. * **Ratio:** The ideal volume of fixative to specimen ratio is **10:1 to 20:1**. * **Penetration:** Formalin penetrates tissue at a rate of approximately 1mm per hour. * **Bone Marrow Biopsy:** While formalin is used for the bone core [1], **Bouin’s fluid** is sometimes preferred by hematopathologists for superior nuclear detail, though formalin remains the standard. **References:** [1] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. (Basic Pathology) introduces the student to key general principles of pathology, both as a medical science and as a clinical activity with a vital role in patient care. Part 2 (Disease Mechanisms) provides fundamental knowledge about the cellular and molecular processes involved in diseases, providing the rationale for their treatment. Part 3 (Systematic Pathology) deals in detail with specific diseases, with emphasis on the clinically important aspects., pp. 257-258. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Bones, Joints, and Soft Tissue Tumors, pp. 1182-1184.

Question 38: What are the histopathological findings in cysticercosis?

A. Retinoblastoma
B. Neurocysticercosis (Correct Answer)
C. Primitive Neuroectodermal Tumor (PNET)
D. Medulloblastoma

Explanation: **Explanation:** **Cysticercosis** is a parasitic infection caused by the larval stage (*Cysticercus cellulosae*) of the pork tapeworm, **Taenia solium** [1]. Humans become accidental intermediate hosts by ingesting eggs through contaminated food or water [1]. **Why the correct answer is right:** When these larvae migrate to the central nervous system, the condition is termed **Neurocysticercosis (NCC)** [3]. It is the most common parasitic infection of the human brain and a leading cause of acquired epilepsy in developing countries. Histopathologically, the cyst contains a fluid-filled bladder and a **scolex** (the head of the parasite) characterized by suckers and hooks. The host tissue reaction varies from minimal inflammation (viable stage) to intense granulomatous inflammation and eventual calcification (dead stage) [2]. **Why the incorrect options are wrong:** * **Retinoblastoma:** A malignant intraocular tumor of childhood originating from immature retinal cells, characterized by Flexner-Wintersteiner rosettes. * **PNET (Primitive Neuroectodermal Tumor):** A group of highly malignant small round blue cell tumors. While they involve the nervous system, they are neoplastic, not parasitic. * **Medulloblastoma:** A common malignant brain tumor in children located in the cerebellum (posterior fossa). It is characterized by Homer-Wright rosettes. **High-Yield NEET-PG Pearls:** * **Imaging:** "Starry sky appearance" on CT/MRI due to multiple calcified cysts. * **Diagnosis:** ELISA for detection of anticysticercal antibodies is the serological test of choice. * **Cytopathology:** FNA of subcutaneous nodules may show fragments of the bladder wall (wavy, fibrillary) and calcareous corpuscles. * **Treatment:** Albendazole or Praziquantel, often administered with steroids to reduce inflammation from dying larvae. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Infectious Diseases, pp. 403-404. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Infectious Diseases, pp. 404-405. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Central Nervous System, pp. 1281-1282.

Question 39: A 25-year-old man presents with right knee pain, described as "popping and catching," for the past 2 years. Examination reveals a reduced range of motion in the right knee without tenderness or swelling. MRI shows a 3-cm mass in the anterior joint space. Arthroscopic partial synovectomy is performed, and a nodular, encapsulated mass is removed. Microscopic examination of the mass reveals synoviocyte-like tumor cells in a hyalinized stroma containing osteoclast-like giant cells and hemosiderin. An antagonist to which of the following growth factors is most useful in treating this lesion?

A. Epidermal Growth Factor (EGF)
B. Fibroblast Growth Factor (FGF)
C. Macrophage Colony-Stimulating Factor (M-CSF) (Correct Answer)
D. Platelet-Derived Growth Factor (PDGF)

Explanation: ### **Explanation** **Diagnosis: Tenosynovial Giant Cell Tumor (TGCT) – Localized Type** The clinical presentation of a young adult with "popping and catching" in the knee, an MRI showing a nodular mass in the joint space, and histology revealing synoviocyte-like cells, **hemosiderin**, and **osteoclast-like giant cells** is classic for **Tenosynovial Giant Cell Tumor (TGCT)** [1]. #### **1. Why M-CSF is the Correct Answer** The pathogenesis of TGCT involves a specific genetic translocation, typically **t(1;2)(p13;q37)**. This translocation fuses the **CSF1 gene** (on chromosome 1) to the **COL6A3 promoter** (on chromosome 2). * This leads to the **overexpression of Colony-Stimulating Factor 1 (CSF-1)**, also known as **Macrophage Colony-Stimulating Factor (M-CSF)**. * The neoplastic cells secrete excess M-CSF, which acts as a chemoattractant for non-neoplastic inflammatory cells (macrophages and giant cells) that express the CSF-1 receptor (CSF1R). * **Pexidartinib**, a CSF1R antagonist, is the FDA-approved systemic therapy for symptomatic TGCT not amenable to surgery. #### **2. Why Other Options are Incorrect** * **A (EGF):** Involved in epithelial cell growth and cancers like EGFR-mutated lung adenocarcinoma, but has no role in TGCT. * **B (FGF):** Associated with achondroplasia (FGFR3) and angiogenesis, but not the primary driver of TGCT. * **D (PDGF):** While PDGF is involved in Dermatofibrosarcoma Protuberans (DFSP) via the *COL1A1-PDGFB* fusion, it is not the driver for TGCT. #### **3. High-Yield Pearls for NEET-PG** * **Two Types of TGCT:** 1. **Localized:** Usually involves small joints (fingers); formerly called Giant Cell Tumor of Tendon Sheath (GCTTS). 2. **Diffuse:** Involves large joints (knee); formerly called **Pigmented Villonodular Synovitis (PVNS)** [1]. * **Histology Triad:** Synovial-like mononuclear cells, hemosiderin-laden macrophages, and multinucleated osteoclast-like giant cells. * **MRI Finding:** "Signal void" or blooming effect on T2-weighted images due to hemosiderin deposition. * **Key Translocation:** **t(1;2)** involving the **CSF1** gene. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Bones, Joints, and Soft Tissue Tumors, pp. 1220-1221.

Question 40: Which stain is used for sebaceous cell carcinoma?

A. Oil Red O (Correct Answer)
B. PAS
C. Methanamine silver
D. KOH

Explanation: **Explanation:** **Sebaceous cell carcinoma** is a rare but aggressive malignant tumor derived from the epithelium of sebaceous glands (commonly in the eyelid) [1]. The hallmark of these cells is the presence of **intracytoplasmic lipid (fat)**. 1. **Why Oil Red O is correct:** Oil Red O is a fat-soluble dye that stains neutral triglycerides and lipids a bright red/orange color. Since sebaceous carcinoma cells contain abundant lipid droplets, Oil Red O is the gold standard for confirming the diagnosis. **Note:** This stain must be performed on **frozen sections**, as routine paraffin processing involves alcohols and xylenes that dissolve lipids, leading to false-negative results. 2. **Why other options are incorrect:** * **PAS (Periodic Acid-Schiff):** Used to detect glycogen and mucopolysaccharides. It is helpful in diagnosing clear cell carcinomas or fungal walls, but not specific for lipids. * **Methenamine Silver (GMS):** Primarily used to visualize fungi (e.g., *Pneumocystis jirovecii*) and basement membranes. * **KOH (Potassium Hydroxide):** A bedside clinical test used to dissolve keratin and visualize fungal hyphae/spores in skin scrapings; it is not a histological stain. **High-Yield Clinical Pearls for NEET-PG:** * **Sudan Black B** is another stain that can be used for lipids/fats. * Sebaceous carcinoma often mimics benign conditions like **chalazion** or chronic blepharoconjunctivitis (masquerade syndrome). * **Pagetoid spread** (individual malignant cells infiltrating the overlying epithelium) is a characteristic histological feature. * **IHC markers:** Adipophilin and Perilipin are modern immunohistochemical markers used for sebaceous differentiation. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Skin, p. 1156.

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