Medical Science 2025 Paper I 50 marks Trace

Q2

(a) Using a well labelled diagram, trace the taste pathway from circumvallate papillae of tongue to the cerebral cortex. 15 marks (b)(i) Enumerate any five tests for the assessment of renal tubular functions. Explain the underlying principle and their interpretation. 10 marks (ii) Explain the role of biotin in intermediary metabolism with suitable examples. Add a note on the mechanism of action of any biotin-antagonist with its application in medicine. 10 marks (c)(i) Define cardiac output and describe the regulation of cardiac output in normal adults. Add a note on Cardiac Index and Cardiac Reserve. 7+3=10 marks (ii) Define Systemic Vascular Resistance and list its determinants. Briefly explain the role of its determinants. 3+2=5 marks

हिंदी में प्रश्न पढ़ें

(a) सुचिहित आरेख का प्रयोग करते हुए स्वाद मार्ग को जिह्वा के परिवृत्त अनुकुरों से प्रमस्तिष्क प्रांतस्था तक अंकित कीजिए। 15 अंक (b)(i) वृक्क नलिकीय क्रियाओं का आकलन करने वाली किन्हीं पाँच जाँचों के नाम गिनाइए। उनके आधारभूत सिद्धांत तथा उनके अर्थनिर्णय (इंटरप्रिटेशन) को समझाइए। 10 अंक (ii) उपयुक्त उदाहरणों के साथ बायोटिन की मध्यवर्ती चयापचय में भूमिका समझाइए। किसी एक बायोटिन-संदमक के कार्य करने की क्रियाविधि, उसके चिकित्सार्थ प्रयोग के साथ प्रस्तुत करते हुए टिप्पणी लिखिए। 10 अंक (c)(i) हृद् निर्गम को परिभाषित कीजिए तथा सामान्य वयस्कों में हृद् निर्गम के विनियमन का वर्णन कीजिए। साथ ही, हृद् सूचकांक तथा हृद् संचिति पर टिप्पणी लिखिए। 7+3=10 अंक (ii) दैहिक वाहिका प्रतिरोध को परिभाषित कीजिए तथा उसके निर्धारकों की सूची बनाइए। उसके निर्धारकों की भूमिका की संक्षेप में व्याख्या कीजिए। 3+2=5 अंक

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Directive word: Trace

This question asks you to trace. The directive word signals the depth of analysis expected, the structure of your answer, and the weight of evidence you must bring.

See our UPSC directive words guide for a full breakdown of how to respond to each command word.

How this answer will be evaluated

Approach

The directive 'trace' demands sequential, anatomically precise narration of neural pathways. Structure your answer with: (a) 15 marks—detailed diagram of taste pathway from circumvallate papillae via cranial nerves VII, IX, solitary nucleus, VPM thalamus to insular cortex; (b)(i) 10 marks—five renal tubular function tests (urine pH, concentrating ability, acidification, glucose/amino acid excretion, PAH extraction) with principles; (b)(ii) 10 marks—biotin's carboxylase role in gluconeogenesis/FA synthesis, plus avidin mechanism; (c)(i) 10 marks—CO definition, Frank-Starling, autonomic regulation, CI and reserve; (c)(ii) 5 marks—SVR definition and determinants. Allocate ~35% time to (a), ~25% each to (b)(i) and (b)(ii), ~20% to (c).

Key points expected

  • (a) Complete taste pathway: circumvallate papillae → taste buds → type III cells → cranial nerve IX (petrosal ganglion) → solitary tract nucleus → VPM nucleus of thalamus → insular cortex (primary) + orbitofrontal cortex (secondary); diagram must show all relay stations and decussation
  • (b)(i) Five renal tubular tests: urine pH/PCO2 gap test, urine concentrating ability (Uosm/Posm, free water clearance), urinary acidification (NH4+ excretion, titratable acid), renal glycosuria/aminoaciduria assessment, PAH extraction ratio; principles based on transport mechanisms and tubular segments
  • (b)(ii) Biotin as CO2 carrier for pyruvate carboxylase, acetyl-CoA carboxylase, propionyl-CoA carboxylase; examples: gluconeogenesis, fatty acid synthesis, odd-chain FA metabolism; avidin-biotin binding with Kd ~10^-15 M, used in ELISA/streptavidin systems and biotinidase deficiency screening
  • (c)(i) Cardiac output = HR × SV; regulation via intrinsic (Frank-Starling, length-tension) and extrinsic (ANS, baroreceptors, hormones); Cardiac Index = CO/BSA (normal 2.5-4.0 L/min/m²); Cardiac Reserve = max CO/resting CO (~4-5×)
  • (c)(ii) SVR = (MAP-CVP)/CO × 80 dynes·sec/cm⁵; determinants: vessel radius (Poiseuille's law), vessel length, blood viscosity; radius most important (T⁴ relationship), regulated by local metabolites and sympathetic tone

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness25%12.5Precise anatomical nomenclature throughout: for (a) correctly identifies petrosal ganglion, solitary tract nucleus, VPM thalamus; for (b)(i) accurately describes proximal vs distal tubular functions; for (b)(ii) correctly names all four biotin-dependent carboxylases with their reactions; for (c) exact formulas for CO, CI, SVR with correct unitsMinor errors in relay station naming (e.g., confusing VII and IX nerve contributions) or incomplete carboxylase list; correct basic formulas but missing units or minor calculation errorsMajor anatomical errors (e.g., taste pathway via trigeminal nerve), confusion between tubular and glomerular function tests, incorrect biotin role, wrong CO/SVR formulas
Clinical correlation20%10For (a) links ageusia/hemiageusia to chorda tympani or thalamic lesions; for (b)(i) connects renal tubular acidosis types to specific test abnormalities; for (b)(ii) discusses biotinidase deficiency in Indian newborns (incidence ~1:60,000) and multiple carboxylase deficiency; for (c) applies CO regulation to shock states and exercise physiologyMentions clinical relevance superficially without specific disease linkage; generic statements about kidney disease or heart failure without test-specific or mechanism-based correlationNo clinical application; fails to mention any disease states or physiological adaptations relevant to the topics covered
Diagram / pathway25%12.5For (a) large, clearly labelled diagram showing: tongue with circumvallate papillae, cranial nerve IX, petrosal ganglion, brainstem with solitary nucleus, thalamic VPM, insular cortex; arrows indicating direction; ipsilateral vs contralateral organization; for other parts, flowcharts or tables where appropriateDiagram present but small or poorly labelled; missing 1-2 key structures (e.g., thalamic relay); adequate but not exemplary presentation of tubular test principles or metabolic pathwaysNo diagram for (a) despite explicit requirement; diagram without labels; or diagram with major anatomical errors; disorganized presentation of test principles or metabolic pathways
Differential / staging15%7.5For (b)(i) differentiates proximal RTA (bicarbonate wasting, low urine pH, hypokalemia) vs distal RTA (urine pH >5.5, hypokalemia/hyperkalemia types) vs hyperkalemic RTA using specific test patterns; for (c) distinguishes high-output vs low-output heart failure and compensatory mechanismsBasic differentiation without detailed test correlation; mentions RTA types or heart failure categories without linking to specific physiological measurementsNo differential approach; conflates proximal and distal tubular dysfunction; fails to distinguish physiological states based on CO or SVR patterns
Management / public-health angle15%7.5For (b)(ii) mentions newborn screening for biotinidase deficiency under Indian RBSK program; for (c) discusses CO monitoring in sepsis management (Surviving Sepsis guidelines) and SVR-guided vasopressor therapy; notes relevance to India's CVD burden and preventive cardiologyGeneric mention of screening or treatment without program specifics; basic awareness of clinical monitoring without guideline reference or public health contextNo management or public health content; fails to mention screening, monitoring, or therapeutic applications of the physiological principles discussed

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