Endocrine Pathology — MCQs

A 25-year-old man presents with 3 months of polyuria and increased thirst. The patient suffered trauma to the base of the skull in a motorcycle accident 4 months ago. A 24-hour urine collection shows polyuria but no evidence of hematuria, glucosuria, or proteinuria. The pathogenesis of polyuria in this patient is most likely caused by a lesion in which of the following areas of the brain?

Endocrine Pathology Indian Medical PG Practice Questions and MCQs

Question 71: A 24-year-old male presented with a swelling on the anterior aspect of the neck. On examination, the swelling was firm and moved with deglutition. A biopsy from the lesion is shown. What is the typical finding and diagnosis?

A. Orphan Annie eye nuclei; Follicular carcinoma of the thyroid
B. Orphan Annie eye nuclei; Papillary carcinoma of the thyroid (Correct Answer)
C. Hurthle cell change; Follicular carcinoma of the thyroid
D. Hurthle cell change; Papillary carcinoma of the thyroid

Explanation: ***Orphan Annie eye nuclei; Papillary carcinoma of the thyroid*** - **Orphan Annie eye nuclei** (ground-glass, empty-appearing nuclei with prominent nucleoli) are the **hallmark histological feature** of papillary thyroid carcinoma. - The clinical presentation of a **firm neck swelling that moves with deglutition** in a young male is classic for papillary thyroid carcinoma, the most common thyroid malignancy. *Orphan Annie eye nuclei; Follicular carcinoma of the thyroid* - While **Orphan Annie eye nuclei** are correctly identified, they are **pathognomonic for papillary carcinoma**, not follicular carcinoma. - **Follicular carcinoma** shows **microfollicular architecture** with **capsular and vascular invasion**, lacking the characteristic nuclear features of papillary carcinoma. *Hurthle cell change; Follicular carcinoma of the thyroid* - **Hurthle cell change** (oncocytic cells with abundant eosinophilic cytoplasm) can occur in various thyroid conditions but is **not the primary histological feature** described here. - The **nuclear morphology** (Orphan Annie eye nuclei) is more specific for diagnosis than cytoplasmic changes like Hurthle cell transformation. *Hurthle cell change; Papillary carcinoma of the thyroid* - While papillary carcinoma can occasionally show **Hurthle cell features**, the **primary diagnostic criterion** is the **nuclear morphology** (Orphan Annie eye nuclei). - **Hurthle cell change** is a **secondary feature** and not the **pathognomonic finding** that defines papillary thyroid carcinoma.

Question 72: Which of the following features is NOT seen in papillary carcinoma of the thyroid?

A. Papillary structures, often complex, with a fibrovascular core
B. Optically clear nuclei with nuclear overlapping
C. Nuclear pseudoinclusions and nuclear grooves
D. Tumor cell immunopositivity for calcitonin (Correct Answer)

Explanation: ### Explanation **Why Option D is the Correct Answer:** Papillary Thyroid Carcinoma (PTC) originates from the **follicular epithelial cells** of the thyroid [3]. Therefore, it expresses markers like Thyroglobulin and TTF-1. **Calcitonin** is a hormone secreted by the **Parafollicular C-cells**. Immunopositivity for calcitonin is the diagnostic hallmark of **Medullary Thyroid Carcinoma (MTC)**, not PTC [4]. **Analysis of Incorrect Options:** * **Option A:** PTC is architecturally characterized by branching papillae containing a central **fibrovascular core** [1]. While a follicular variant exists, the presence of complex papillae is a classic morphological feature [1], [3]. * **Option B:** The nuclei in PTC are distinctive [1]. They appear "empty" or "optically clear" due to finely dispersed chromatin, famously known as **Orphan Annie eye nuclei** [1], [2]. Nuclear crowding and overlapping are also characteristic. * **Option C:** These are the most reliable diagnostic features of PTC. **Nuclear grooves** result from longitudinal folding of the nuclear membrane, while **pseudoinclusions** (cytoplasmic invaginations into the nucleus) appear as sharp, round "holes" within the nucleus [1]. **NEET-PG Clinical Pearls:** * **Most Common:** PTC is the most common thyroid malignancy (85%) and has an excellent prognosis [2]. * **Risk Factor:** Prior exposure to ionizing radiation is a major risk factor. * **Psammoma Bodies:** These are concentric calcified laminated structures found in ~50% of PTC cases; they are highly suggestive of the diagnosis [1]. * **Genetics:** Frequently associated with **BRAF mutations** (specifically V600E) and **RET/PTC rearrangements** [3]. * **Spread:** PTC characteristically spreads via **lymphatics** to cervical lymph nodes, whereas Follicular Carcinoma spreads hematogenously [2]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Endocrine System, p. 1099. [2] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Liver And Biliary System Disease, pp. 429-430. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Endocrine System, pp. 1099-1100. [4] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Endocrine System, pp. 1102-1103.

Question 73: Psammoma bodies in a thyroid lesion or the adjacent metastatic lymph nodes increase the possibility of which diagnosis?

A. Papillary carcinoma (Correct Answer)
B. Follicular carcinoma
C. Anaplastic carcinoma
D. Hurthle cell carcinoma

Explanation: **Explanation:** **Psammoma bodies** are concentric, laminated calcified structures (dystrophic calcification) that are a hallmark histological feature of **Papillary Thyroid Carcinoma (PTC)**. They are found in approximately 40–50% of PTC cases, occurring within the fibrovascular cores of the papillae or in the surrounding stroma and lymphatics [1]. Their presence in a thyroid fine-needle aspiration (FNA) or a cervical lymph node is highly suggestive of PTC, even if the primary tumor is occult [1], [2]. **Why other options are incorrect:** * **Follicular Carcinoma:** Characterized by follicular architecture and vascular/capsular invasion [3]. It typically lacks papillae and psammoma bodies. * **Anaplastic Carcinoma:** A highly aggressive, undifferentiated tumor showing pleomorphic giant cells or spindle cells [2]. While it may show necrosis, psammoma bodies are not a characteristic feature. * **Hurthle Cell Carcinoma:** A variant of follicular neoplasm defined by an abundance of mitochondria-rich eosinophilic cells (oncocytes) [4]. It does not typically form psammoma bodies. **High-Yield NEET-PG Pearls:** 1. **Mnemonic for Psammoma Bodies:** **PSaMM**oma – **P**apillary (Thyroid, Renal, Serous Ovarian), **S**erous cystadenocarcinoma of ovary, **M**eningioma, **M**esothelioma. 2. **Nuclear Features of PTC:** The diagnosis of PTC is based on nuclear morphology (**Orphan Annie eye nuclei**, nuclear grooves, and pseudo-inclusions), not just the presence of papillae or psammoma bodies [1], [2]. 3. **Lymphatic Spread:** PTC primarily spreads via lymphatics to cervical nodes, whereas Follicular carcinoma spreads hematogenously [2]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Endocrine System, p. 1099. [2] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Liver And Biliary System Disease, pp. 429-430. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Endocrine System, pp. 1100-1101. [4] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Endocrine System, pp. 1096-1097.

Question 74: What is characteristic of parathyroid carcinoma?

A. 5-10% incidence
B. Increased parathormone, decreased calcium in bone (Correct Answer)
C. Cytology is diagnostic
D. Metastasis is essential

Explanation: **Explanation:** Parathyroid carcinoma is a rare cause of primary hyperparathyroidism. The correct answer reflects the fundamental pathophysiology of the disease: **excessive secretion of Parathyroid Hormone (PTH).** [1] 1. **Why Option B is correct:** Parathyroid carcinoma causes extreme elevations in PTH levels (often >10 times the upper limit of normal). PTH stimulates osteoclasts to resorb bone matrix, leading to the mobilization of calcium into the bloodstream. This results in **decreased calcium in the bone** (manifesting as osteitis fibrosa cystica) and profound hypercalcemia. [1] 2. **Why other options are incorrect:** * **Option A:** Parathyroid carcinoma is very rare, accounting for **<1%** of cases of primary hyperparathyroidism, not 5-10%. * **Option C:** Cytology (FNAC) is **not diagnostic** and is generally contraindicated. It cannot reliably distinguish between a benign adenoma and a carcinoma. Furthermore, FNAC carries a risk of "capsular seeding," potentially spreading malignant cells. * **Option D:** While metastasis is a definitive sign of malignancy, it is **not "essential"** for the initial diagnosis. Diagnosis is primarily based on local invasion into surrounding structures (thyroid, recurrent laryngeal nerve) or specific histological features like capsular/vascular invasion and high mitotic figures. **NEET-PG High-Yield Pearls:** * **Clinical Presentation:** Patients often present with a **palpable neck mass** and severe symptomatic hypercalcemia (Calcium often >14 mg/dL). * **Genetic Association:** Strongly linked to mutations in the **CDC73 (HRPT2)** gene, which encodes the protein **Parafibromin**. Loss of parafibromin expression is a key IHC marker. * **Gold Standard for Diagnosis:** Histological evidence of **local invasion** or **metastasis** (most commonly to regional lymph nodes, lungs, or liver). [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Endocrine System, pp. 1105-1106. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Endocrine System, pp. 1105-1106.

Question 75: Hashimoto's disease is:

A. Agranulomatous thyroiditis
B. An autoimmune thyroiditis (Correct Answer)
C. Fibrous thyroiditis
D. A viral infection of the thyroid gland

Explanation: **Explanation:** **Hashimoto’s Thyroiditis** (Chronic Lymphocytic Thyroiditis) is the most common cause of hypothyroidism in iodine-sufficient regions [2]. It is primarily an **autoimmune disorder** characterized by the breakdown of self-tolerance to thyroid autoantigens [5]. The pathogenesis involves a combination of **Type IV hypersensitivity** (T-cell mediated cytotoxicity) and **Type II hypersensitivity** (Antibody-dependent cell-mediated cytotoxicity), leading to the progressive destruction of thyroid follicles [5]. **Analysis of Options:** * **Option B (Correct):** It is an autoimmune disease marked by the presence of circulating autoantibodies, most notably **Anti-TPO (Antithyroid peroxidase)** and **Anti-Tg (Antithyroglobulin)**. * **Option A:** Granulomatous thyroiditis refers to **de Quervain’s thyroiditis**, which is characterized by multinucleated giant cells and is typically triggered by a viral infection. * **Option C:** Fibrous thyroiditis refers to **Riedel’s thyroiditis**, a rare condition where the thyroid parenchyma is replaced by dense fibrous tissue (associated with IgG4-related disease). * **Option D:** Viral infections are associated with Subacute (de Quervain’s) thyroiditis, not Hashimoto’s. **High-Yield Clinical Pearls for NEET-PG:** * **Histology:** Look for a dense **lymphocytic infiltrate** with well-developed **germinal centers** [1], [4]. * **Hürthle Cells:** These are transformed follicular epithelial cells with abundant, granular, eosinophilic cytoplasm (due to mitochondrial metaplasia) [1], [4]. * **HLA Association:** Strongly linked with **HLA-DR3** and **HLA-DR5**. * **Risk of Malignancy:** Patients have an increased risk of developing **B-cell Non-Hodgkin Lymphoma** (specifically MALToma) and Papillary Thyroid Carcinoma [3]. * **Clinical Sign:** Often presents as a painless, diffuse enlargement of the thyroid (goiter) in middle-aged women [1]. **References:** [1] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Liver And Biliary System Disease, pp. 427-428. [2] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Liver And Biliary System Disease, pp. 426-427. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 235-236. [4] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Endocrine System, pp. 1090-1091. [5] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Endocrine System, pp. 1089-1090.

Question 76: What is the prognosis of thyroid cancer?

A. Papillary
B. Medullary
C. Anaplastic (Correct Answer)
D. Follicular

Explanation: **Explanation:** The prognosis of thyroid cancer is primarily determined by its histological subtype [2]. **Anaplastic Thyroid Carcinoma (ATC)** is the correct answer because it is one of the most aggressive solid tumors in humans, carrying a near-universal mortality rate [1]. **Why Anaplastic is the Correct Answer:** Anaplastic carcinoma is an undifferentiated tumor typically seen in elderly patients [2]. It presents as a rapidly enlarging neck mass that invades local structures (trachea, esophagus), leading to dyspnea and dysphagia [1]. Most patients have distant metastases at the time of diagnosis. The 5-year survival rate is less than 5%, making it the thyroid cancer with the **worst prognosis** [2]. **Analysis of Incorrect Options:** * **Papillary Carcinoma (A):** This is the most common thyroid cancer [2]. It has an **excellent prognosis** (10-year survival >95%) and typically spreads via lymphatics [2]. * **Follicular Carcinoma (D):** This is the second most common type. While more aggressive than papillary (spreads hematogenously), it still carries a **good prognosis** if caught early [2]. * **Medullary Carcinoma (B):** Derived from parafollicular C-cells (secreting Calcitonin) [4], its prognosis is **intermediate**—worse than papillary/follicular but significantly better than anaplastic [2]. **NEET-PG High-Yield Pearls:** * **Prognosis Order (Best to Worst):** Papillary > Follicular > Medullary > Anaplastic [2]. * **Psammoma bodies** are a hallmark of Papillary carcinoma. * **Amyloid stroma** (Congo Red positive) is the classic finding in Medullary carcinoma. * **Orphan Annie eye nuclei** and **nuclear grooves** are diagnostic for Papillary carcinoma [3]. * Anaplastic carcinoma often stains positive for **PAX8** but is usually negative for Thyroglobulin [1]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Endocrine System, pp. 1101-1102. [2] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Liver And Biliary System Disease, pp. 429-430. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Endocrine System, p. 1099. [4] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Liver And Biliary System Disease, pp. 428-429.

Question 77: A 27-year-old female presented with a long-standing 2 cm x 2 cm nodule in the right lobe of her thyroid, for which she underwent a right hemithyroidectomy. What is the most likely histopathological finding given these clinical details?

A. Adenomatous goiter
B. Papillary carcinoma (Correct Answer)
C. Follicular adenoma
D. Graves disease

Explanation: **Explanation:** The correct answer is **Papillary Thyroid Carcinoma (PTC)**. In the context of NEET-PG, any solitary thyroid nodule in a young female (20–40 years) is considered a "red flag." Statistically, PTC is the most common thyroid malignancy (85% of cases) and frequently presents as a painless, slow-growing solitary nodule in an otherwise euthyroid patient [2]. **Why the other options are less likely:** * **Adenomatous Goiter:** This typically presents as a multinodular enlargement of the thyroid (Multinodular Goiter) rather than a discrete, long-standing solitary nodule [1]. * **Follicular Adenoma:** While it presents as a solitary nodule, it is less common than PTC [2]. Furthermore, the "most likely" examiner expectation for a solitary nodule in a young female is malignancy, specifically PTC. * **Graves Disease:** This is a functional disorder presenting with diffuse thyroid enlargement (goiter) and clinical features of hyperthyroidism (tachycardia, tremors, exophthalmos), not a localized 2 cm nodule. **High-Yield Clinical Pearls for NEET-PG:** * **Diagnosis:** Fine Needle Aspiration Cytology (FNAC) is the investigation of choice for thyroid nodules [1]. * **Hallmark Histology of PTC:** Look for **Orphan Annie eye nuclei** (optically clear), **Psammoma bodies** (laminated calcifications), and **Nuclear grooves/pseudoinclusions** [1]. * **Prognosis:** PTC has an excellent prognosis despite a tendency for early lymphatic spread to cervical lymph nodes [2]. * **Risk Factor:** Prior exposure to ionizing radiation is a significant risk factor for developing PTC. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Endocrine System, pp. 1095-1100. [2] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Liver And Biliary System Disease, pp. 428-430.

Question 78: A 40-year-old female patient complains of a painless anterior neck mass. She has a history of hypothyroidism. Her blood work suggests increased TSH levels and decreased T3 and T4 levels. Circulating thyroid autoantibodies are also confirmed. What is the most likely diagnosis?

A. De Quervain's thyroiditis
B. Riedel's thyroiditis
C. Hashimoto’s thyroiditis (Correct Answer)
D. None of the above

Explanation: ### Explanation **Hashimoto’s Thyroiditis (Correct Answer)** Hashimoto’s thyroiditis (Chronic Lymphocytic Thyroiditis) is the most common cause of hypothyroidism in iodine-sufficient regions [2]. It is an autoimmune disorder characterized by the destruction of thyroid follicles by CD8+ cytotoxic T-cells and cytokine-mediated cell death. [1] * **Clinical Presentation:** Typically presents in middle-aged females as a painless, diffuse enlargement of the thyroid (goiter). * **Lab Findings:** The classic triad includes **primary hypothyroidism** (High TSH, Low T3/T4) and the presence of **circulating autoantibodies** (Anti-TPO and Anti-thyroglobulin) [1]. * **Pathology:** Histologically, it shows intense lymphocytic infiltration with germinal centers and **Hürthle cells** (metaplastic eosinophilic, granular cells) [1], [2]. **Why Incorrect Options are Wrong:** * **De Quervain’s Thyroiditis (Subacute Granulomatous):** This is typically post-viral and presents with a **painful, tender** thyroid gland. It usually follows a triphasic course (hyperthyroidism → hypothyroidism → recovery) and is associated with an elevated ESR. * **Riedel’s Thyroiditis:** A rare manifestation of IgG4-related disease characterized by extensive **fibrosis** that extends beyond the thyroid capsule into adjacent neck structures, making the gland "stony hard" and fixed [1]. **High-Yield NEET-PG Pearls:** * **HLA Association:** Strongly linked with **HLA-DR3** and **HLA-DR5**. * **Increased Risk:** Patients have an increased risk of developing **B-cell Non-Hodgkin Lymphoma** (specifically MALT lymphoma) and Papillary Thyroid Carcinoma [1]. * **Hurthle Cells:** Also known as Askanazy cells; they are follicular epithelial cells with abundant mitochondria [1], [2]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Endocrine System, pp. 1090-1092. [2] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Liver And Biliary System Disease, pp. 426-428.

Question 79: What is the pathognomic feature of hyperparathyroidism?

A. Osteopenia
B. Loss of lamina dura
C. Brown's tumor
D. Subperiosteal resorption of phalanges (Correct Answer)

Explanation: **Explanation:** Hyperparathyroidism (HPT) is characterized by excessive secretion of Parathyroid Hormone (PTH), which stimulates osteoclast activity, leading to increased bone resorption [1]. **Why Subperiosteal Resorption is the Correct Answer:** Subperiosteal resorption of the phalanges (specifically the radial aspect of the middle phalanges) is considered the **pathognomonic** radiological hallmark of hyperparathyroidism [1]. It is the most sensitive and specific sign of the disease. PTH acts on the subperiosteal bone, which has a high turnover rate, making this the earliest and most diagnostic site for identifying hyperparathyroid bone disease (**Osteitis Fibrosa Cystica**) [2]. **Analysis of Incorrect Options:** * **A. Osteopenia:** This is a non-specific finding characterized by decreased bone mineral density. It occurs in many conditions, including osteoporosis and osteomalacia, and is not unique to HPT [1]. * **B. Loss of lamina dura:** This refers to the disappearance of the cortical bone lining the tooth sockets. While it is a classic sign of HPT, it is **not pathognomonic** as it can also be seen in Paget’s disease and severe osteoporosis. * **C. Brown’s tumor:** These are non-neoplastic reactive lesions (masses of osteoclasts and fibrous tissue with hemorrhage) that occur in advanced HPT [2]. While highly suggestive, they are a late manifestation and less specific than subperiosteal resorption. **NEET-PG High-Yield Pearls:** * **Osteitis Fibrosa Cystica (von Recklinghausen disease of bone):** The classic constellation of skeletal changes in HPT [2]. * **Salt and Pepper Skull:** Granular decalcification of the skull seen on X-ray. * **Rugger Jersey Spine:** Characteristic of secondary hyperparathyroidism (Renal Osteodystrophy) [3]. * **Biochemical Triad:** Hypercalcemia, Hypophosphatemia, and elevated PTH (in Primary HPT) [4]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Bones, Joints, and Soft Tissue Tumors, p. 1194. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Endocrine System, pp. 1105-1106. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Bones, Joints, and Soft Tissue Tumors, pp. 1194-1195. [4] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Osteoarticular And Connective Tissue Disease, pp. 667-668.

Question 80: A 25-year-old man presents with 3 months of polyuria and increased thirst. The patient suffered trauma to the base of the skull in a motorcycle accident 4 months ago. A 24-hour urine collection shows polyuria but no evidence of hematuria, glucosuria, or proteinuria. The pathogenesis of polyuria in this patient is most likely caused by a lesion in which of the following areas of the brain?

A. Adenohypophysis
B. Brain stem
C. Mammillothalamic tract
D. Neurohypophysis (Correct Answer)

Explanation: ### Explanation **Correct Option: D (Neurohypophysis)** The clinical presentation of polyuria and polydipsia following head trauma is classic for **Central Diabetes Insipidus (CDI)** [1]. The pathogenesis involves a deficiency of **Antidiuretic Hormone (ADH/Vasopressin)**. ADH is synthesized in the supraoptic and paraventricular nuclei of the hypothalamus and transported via axons to the **neurohypophysis (posterior pituitary)**, where it is stored and released. Trauma to the skull base or pituitary stalk disrupts this transport or release mechanism, leading to an inability of the kidneys to concentrate urine, resulting in the excretion of large volumes of dilute urine [1], [2]. **Analysis of Incorrect Options:** * **A. Adenohypophysis:** The anterior pituitary produces hormones like GH, ACTH, and TSH. While trauma can cause panhypopituitarism, isolated polyuria is specifically a result of posterior pituitary (neurohypophysis) dysfunction [2]. * **B. Brain stem:** This area controls vital functions (respiration, heart rate) and cranial nerve nuclei. It does not play a direct role in ADH production or water homeostasis. * **C. Mammillothalamic tract:** This tract is part of the limbic system (Papez circuit) involved in memory. Lesions here (e.g., Wernicke-Korsakoff syndrome) cause memory deficits, not polyuria. **High-Yield Clinical Pearls for NEET-PG:** * **Triad of CDI:** Polyuria, polydipsia, and low urine specific gravity (<1.005) [1]. * **Water Deprivation Test:** In CDI, urine osmolality remains low after water restriction but **increases by >50%** following administration of exogenous desmopressin (differentiating it from Nephrogenic DI). * **Most common cause of CDI:** Idiopathic (30%), followed by tumors (craniopharyngioma) and trauma [2]. * **"Bright Spot" on MRI:** The normal posterior pituitary shows a high-intensity signal (T1) on MRI; the absence of this "bright spot" is characteristic of CDI. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Endocrine System, pp. 1084-1085. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Endocrine System, pp. 1083-1084.

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