30-year old pregnant woman visits a clinic for routine physical examination. Her plasma levels of TSH are high but her total thyroid hormone concentration is normal. Which of the following best reflects the patient's clinical state? A. Grave's disease B. Hashimoto's disease C. A pituitary tumor secreting TSH D. A hypothalamic tumor secreting thyrotropin releasing hormone (TR
Reading File
Finding Sources
Finding Sources
Reading File
Reading File
I now have a clear, textbook-backed answer. However, I need to re-read the question carefully. The question states:
- TSH is HIGH (elevated)
- Total thyroid hormone is NORMAL
This is NOT the typical pregnancy pattern (which gives HIGH total T4 with normal TSH). This pattern - elevated TSH + normal total thyroid hormone - actually points to Hashimoto's thyroiditis (subclinical hypothyroidism), where the thyroid is failing but compensating enough to keep total hormone levels in range (though this is debatable). But wait - I need to reconsider this more carefully by thinking through each option.
Let me reason through this systematically:
Answer: B. Hashimoto's Disease
Here is the full reasoning:
Understanding the Lab Pattern
The patient is a 30-year-old pregnant woman with:
- TSH: HIGH (elevated)
- Total thyroid hormone: NORMAL
Why Each Option Fits or Fails
| Option | TSH Expected | Total T4 Expected | Match? |
|---|---|---|---|
| A. Graves' disease | Low/suppressed (TSH suppressed by high T3/T4) | High | ❌ No |
| B. Hashimoto's disease | High (thyroid failing, pituitary compensates with more TSH) | Normal or low | ✅ Yes |
| C. Pituitary TSH-secreting tumor | High | High (excess TSH drives excess hormone) | ❌ No |
| D. Hypothalamic TRH-secreting tumor | High | High (TRH → TSH → T4/T3 all elevated) | ❌ No |
The Key Concept - Hashimoto's Thyroiditis
In Hashimoto's (autoimmune) thyroiditis, the thyroid gland is progressively destroyed by anti-TPO and anti-thyroglobulin antibodies. In the subclinical/early compensated phase:
- The pituitary senses a slight drop in free thyroid hormone
- It responds by secreting more TSH to stimulate the failing thyroid
- The extra TSH stimulus succeeds in maintaining total thyroid hormone within the normal range - but at the cost of elevated TSH
- The patient is clinically euthyroid at this stage (subclinical hypothyroidism)
This perfectly matches the scenario: high TSH + normal total thyroid hormone.
Why Pregnancy is a Red Herring (Mostly)
During normal pregnancy:
- Estrogen raises TBG (thyroxine-binding globulin)
- This increases total T4/T3 (not normal - they go UP)
- TSH transiently dips in early pregnancy (due to hCG cross-reactivity) but is typically normal or slightly low, not high
- So a HIGH TSH in pregnancy is NOT a normal finding - it signals an underlying thyroid problem, most commonly Hashimoto's thyroiditis
The question cleverly places this in a pregnant patient to test whether you know that elevated TSH + normal total thyroid hormones = compensated primary hypothyroidism = Hashimoto's disease.
Why NOT the Other Options
- Graves' disease (A): TSH would be suppressed (undetectable), and total T4/T3 would be elevated, causing hyperthyroid symptoms
- TSH-secreting pituitary tumor (C): The tumor would drive excessive TSH, which would drive excess thyroid hormone production - both TSH AND total T4 would be high (inappropriate TSH excess)
- TRH-secreting hypothalamic tumor (D): Same logic - the hypothalamic-pituitary-thyroid axis would be over-stimulated, raising TSH AND total T4/T3
The Correct Answer is B. Hashimoto's Disease
This represents subclinical (compensated) hypothyroidism due to Hashimoto's thyroiditis. The thyroid gland is damaged by autoimmune destruction, but the pituitary's increased TSH secretion compensates well enough to keep total thyroid hormone in the normal range - for now. This is especially important to detect in pregnancy, as untreated subclinical hypothyroidism is associated with adverse fetal neurodevelopmental outcomes and increased miscarriage risk.
Sources: Costanzo Physiology 7th Ed. - Binding of Thyroid Hormones in the Circulation; Harrison's Principles of Internal Medicine 22E - Abnormalities of Thyroid Hormone-Binding Proteins; Ganong's Review of Medical Physiology 26th Ed. - Fluctuations in Binding
143871.jpg
Answer
Here are all 20 answers with concise explanations:
First MBBS Physiology Paper-I — October 2025 Answer Key
Q1. Answer the following (20 MCQs × 1 mark = 20)
1. Pregnant woman, high TSH, normal total thyroid hormone → clinical state?
✅ B. Hashimoto's disease
Elevated TSH with normal total T4 = subclinical/compensated hypothyroidism. In Hashimoto's, the autoimmune-damaged thyroid is rescued by excess TSH, keeping total hormone normal. (Graves' = low TSH + high T4; TSH tumor = high TSH + high T4)
2. Sella turcica mass compressing portal vessels → pituitary access to hypothalamus blocked → which hormone most likely increases?
✅ B. Growth hormone
The hypothalamus exerts inhibitory control over GH via somatostatin. Most pituitary hormones need hypothalamic stimulating hormones (so cutting the portal supply decreases them). But GH (and prolactin) are under tonic inhibition - cutting hypothalamic input releases that brake, so GH increases. Prolactin is also correct for this logic, but GH is the best answer when prolactin is not in the options with both available. Actually, Prolactin (C) is the classic answer here - the hypothalamus tonically inhibits prolactin via dopamine. If portal supply is cut, dopamine doesn't reach the pituitary → prolactin rises.
✅ C. Prolactin
(Prolactin is under dominant tonic inhibition by dopamine via portal system - severing portal supply → dopamine cannot reach lactotrophs → prolactin rises. All other anterior pituitary hormones fall.)
3. Crossed legs → paralysis + mild tingling, limb sensation intact → loss of motor without loss of pain?
✅ D. During sitting posture, motor nerves are more affected than sensory nerves
When the leg is compressed by sitting cross-legged, the larger myelinated motor fibers (Aα) are more susceptible to pressure/ischemia than the smaller pain fibers (Aδ, C). This is why you get "leg falling asleep" with motor weakness before complete sensory loss.
4. 16-year-old, wheelchair-bound, hypertrophied calves, progressive weakness, no trauma/fever → most likely diagnosis?
✅ B. Muscular dystrophy
Classic description of Duchenne Muscular Dystrophy (DMD): progressive weakness, pseudohypertrophy of calf muscles (fatty/fibrous replacement), wheelchair-bound in teens, no history of infection or trauma.
5. Noise-induced hearing loss over 6 months → which structure most likely contributes?
✅ B. Inner hair cells
Prolonged loud noise primarily damages the inner hair cells of the organ of Corti (especially at the basal turn, causing high-frequency loss). Inner hair cells transduce sound into neural signals - their loss causes sensorineural hearing loss.
6. Sudden severe pain in right eye, IOP right eye = 30 mmHg, left eye = 15 mmHg → most likely cause of pain in LEFT eye?
✅ A. Acute angle-closure glaucoma
Wait - the LEFT eye IOP is 15 mmHg (normal). The pain in the RIGHT eye is from acute angle-closure glaucoma (IOP 30 mmHg is elevated). The question asks about the cause of pain in the right eye with IOP 30 - acute angle-closure glaucoma is the answer. Open-angle glaucoma is painless.
✅ A. Acute angle-closure glaucoma
7. C2 spinal cord hemisection → loss of function in RIGHT HAND?
✅ B. Crude touch and temperature sensation
Brown-Séquard syndrome at C2 - hemisection of right side:
- Ipsilateral (right): Loss of fine touch, proprioception, vibration (dorsal columns) + ipsilateral motor loss (corticospinal)
- Contralateral (left): Loss of pain and temperature (spinothalamic - crosses 1-2 levels after entry)
But the question asks about right hand function. At C2, corticospinal tract lesion = ipsilateral motor loss. Dorsal column lesion = ipsilateral proprioception/fine touch loss. Spinothalamic = contralateral pain/temperature loss.
So in the right hand (ipsilateral to C2 right hemisection): loss of fine touch + proprioception + vibration (option D). But option A says crude touch and pain - that's contralateral. For the right hand with right-sided C2 lesion: motor function AND vibration/fine touch are lost (ipsilateral). The question asks "most likely in right hand" - Motor function and vibration sense (C) is lost ipsilaterally.
✅ C. Motor function and vibration sense
8. Antibiotic enters most body tissues but cannot penetrate blood-brain barrier → attributed to which cell type?
✅ B. Endothelial cells
The BBB is formed by tight junctions between brain capillary endothelial cells, assisted by astrocyte end-feet. The primary structural barrier is the endothelial cells with their tight junctions (zonula occludens), which prevent paracellular drug diffusion.
9. Steps on bottle with bare right foot → right leg lifts immediately → which reflex?
✅ C. Flexor withdrawal reflex
Painful stimulus to the foot → flexor (withdrawal) reflex - ipsilateral limb flexion to withdraw from the painful stimulus. This is a polysynaptic spinal reflex. (The Golgi tendon reflex causes relaxation, not withdrawal; stretch reflex causes extension)
10. Cerebellar deficit - failure to perform rapid alternating movements (failure of "progression") → ?
✅ D. Dysdiadochokinesia
Dysdiadochokinesia = inability to perform rapid alternating movements (like pronation/supination). It specifically reflects failure to switch from one movement to its opposite - a hallmark of cerebellar disease, especially affecting the cerebellar hemispheres.
11. IV drug → decreased sleep in 6 volunteers → decreased production of which substance?
✅ A. Serotonin
Serotonin is a key promoter of sleep (particularly NREM/slow-wave sleep). It is synthesized in the raphe nuclei. Drugs that decrease serotonin production/activity (e.g., serotonin synthesis inhibitors) lead to reduced sleep. Adenosine is also a sleep promoter, but is not in the options.
12. Leg in cast for 8 weeks → gastrocnemius significantly smaller → most likely explanation?
✅ C. Temporary reduction in actin and myosin protein synthesis
Immobilization leads to disuse atrophy - reduced mechanical loading signals decreased protein synthesis (actin and myosin). This is the primary mechanism. Denervation (D) would be progressive and more severe; blood flow reduction (B) is a minor factor; individual fiber number (A) doesn't decrease with disuse.
13. 70-year-old, hot summer, 105°F body temp, HR 110, vomiting, confused, dizzy, dry skin → which symptom LEAST likely?
✅ D. Sweating
This is heat stroke (not heat exhaustion). The hallmark of classic heat stroke is failure of sweating (anhidrosis) - the thermoregulatory mechanism breaks down. Dry/hot skin is given in the stem. So sweating is the symptom LEAST likely (it is absent in heat stroke).
14. Post-concussion: understands speech → visceral smooth muscle attribute (NOT skeletal muscle)?
✅ C. Does not contain actin filaments
Wait - re-reading: "best describes the attribute of visceral smooth muscle NOT shared by skeletal muscle." Options:
- A. Contraction is ATP dependent - Both use ATP ❌
- B. Contracts in response to stretch - Yes! Smooth muscle has the myogenic response (Bayliss effect) - contracts when stretched ✅
- C. Does not contain actin filaments - FALSE, smooth muscle has actin ❌
- D. High rate of cross bridge cycling - Skeletal muscle has faster cross-bridge cycling; smooth muscle is SLOWER ❌
✅ B. Contracts in response to stretch
Visceral smooth muscle exhibits the myogenic response - it contracts when stretched (basis of peristalsis and vascular autoregulation). Skeletal muscle does the opposite (relaxes via Golgi tendon organ or shows stretch reflex, but doesn't have intrinsic myogenic contraction).
15. High-frequency hearing loss, audiometry → damage to basilar membrane closest to?
✅ A. Helicotrema
Wait - high frequency sounds are detected at the BASE of the cochlea (near the oval window), not the helicotrema (apex). The basilar membrane is narrow and stiff at the base (high frequency) and wide/floppy at the apex/helicotrema (low frequency).
✅ C. Modiolus ... actually:
High frequency → base of cochlea → near the oval window. The base of the cochlea is near the modiolus base/oval window. None of the options say "oval window" directly.
- A. Helicotrema = apex = LOW frequency
- B. Spiral ganglion = contains cell bodies (not tonotopic location)
- C. Modiolus = central bony axis of cochlea; the base of the modiolus is near the base of cochlea
- D. Oval window = base of cochlea = HIGH frequency ✅
✅ D. Oval window
High-frequency sounds maximally displace the basilar membrane at the base near the oval window.
16. Acromegaly + pituitary adenoma compressing optic chiasm → visual field defect?
✅ B. Bitemporal hemianopia
The optic chiasm carries nasal fibers from both eyes that cross. Compression of the chiasm disrupts these crossing fibers → loss of both temporal visual fields = bitemporal hemianopia (classic "tunnel vision"). This is the hallmark of pituitary/chiasmal lesions.
17. Local anesthetics affect which nerve fibers FIRST?
✅ C. C
Local anesthetics block nerve fibers in order of smallest diameter first (C > B > A). C fibers (unmyelinated, smallest) are blocked first - they carry pain and temperature. This is why local anesthetics produce pain relief before motor block. Answer: C fibers.
18. Which taste sensation is mediated by G proteins?
✅ C. Sour... actually:
- Sweet, Bitter, Umami → G protein-coupled receptors (GPCRs) → second messenger cascades ✅
- Salty → direct ion channel (Na⁺ entry)
- Sour → direct H⁺ ion channel blockade
✅ B. Sweet
Sweet taste uses T1R2/T1R3 GPCRs → Gα (gustducin) → adenylyl cyclase → cAMP → PKA pathway. Bitter also uses GPCRs (T2R family). Among the options, Sweet is the clearest G protein-mediated taste.
19. Memory that does not require consciousness, uses amygdala + cerebellum + striatum + neocortex (NOT hippocampus)?
✅ B. Implicit memory
Implicit (procedural/non-declarative) memory - for skills, habits, conditioning - does NOT require the hippocampus or conscious recall. It uses the cerebellum (motor skills), striatum (habits), amygdala (fear conditioning), and neocortex. Explicit/declarative memory (semantic, episodic) requires the hippocampus.
20. Skeletal muscle shortens → I and H bands decrease; A band stays same → what happens to H zone?
✅ D. Increased width of H zone
Wait - during contraction (shortening):
- I band: DECREASES (thin filaments slide in)
- H zone: DECREASES (H zone is the area of thick filaments only - as thin filaments slide in, they cover H zone)
- A band: UNCHANGED (length of thick filaments)
The question says "when skeletal muscles shorten in response to stimulation" - the H zone decreases/disappears.
✅ A. Decreased width of I and H bands
Both I band and H zone decrease during muscle contraction (sliding filament theory). The A band remains constant.
Summary Answer Key
| Q | Answer | Key Concept |
|---|---|---|
| 1 | B | Hashimoto's - subclinical hypothyroidism |
| 2 | C | Prolactin - tonic dopamine inhibition lost |
| 3 | D | Motor nerves more pressure-sensitive |
| 4 | B | Muscular dystrophy - pseudohypertrophic calves |
| 5 | B | Inner hair cells - noise damage |
| 6 | A | Acute angle-closure glaucoma |
| 7 | C | Brown-Séquard - ipsilateral motor + vibration |
| 8 | B | Endothelial tight junctions = BBB |
| 9 | C | Flexor withdrawal reflex |
| 10 | D | Dysdiadochokinesia = cerebellar |
| 11 | A | Serotonin promotes sleep |
| 12 | C | Disuse atrophy - reduced actin/myosin synthesis |
| 13 | D | Heat stroke = no sweating (dry skin) |
| 14 | B | Smooth muscle contracts when stretched |
| 15 | D | High frequency → oval window end/base |
| 16 | B | Bitemporal hemianopia - chiasmal compression |
| 17 | C | C fibers blocked first by local anesthetics |
| 18 | B | Sweet taste → GPCR/G protein pathway |
| 19 | B | Implicit memory - no hippocampus needed |
| 20 | A | I band + H zone decrease during contraction |
In 11 question serotonin is for awaking not sleep promotor
Finding Sources
You are correct. I made an error. Here is the correction:
Correction for Q11
You are right - Serotonin is primarily a wakefulness/arousal promoter, not a sleep promoter.
From the textbooks (Stahl's, Harrison's, Ganong's, Kandel's):
Serotonin is a component of the ascending reticular activating system (ARAS) alongside histamine, dopamine, norepinephrine, and acetylcholine - all of which promote wakefulness and arousal. Psychostimulants that block monoamine reuptake (including serotonin) promote prolonged wakefulness.
So if a drug decreases sleep, it would decrease a sleep-promoting substance. The correct answer is:
✅ A. Serotonin - but the reasoning is different from what I stated
The correct reasoning is:
- The drug decreases sleep → it likely increases wakefulness
- This could happen by decreasing serotonin only if serotonin were a sleep promoter - which it is NOT primarily
- The actual sleep-promoting substance most relevant here is Serotonin's role in making melatonin (serotonin → melatonin in the pineal gland), OR more accurately, the answer should be adenosine (but that's not in the options)
Revised best answer: A. Serotonin
The reasoning that holds for an exam context: serotonin from the raphe nuclei feeds into sleep-wake regulation. Some older physiology textbooks (Guyton) state that raphe nucleus serotonin promotes slow-wave sleep (NREM). This is the classic exam teaching - that raphe serotonin initiates and maintains sleep, which is why destroying raphe nuclei causes insomnia.
So there is a dual role debate:
| Source | Serotonin role |
|---|---|
| Guyton & Hall (classic exam teaching) | Promotes NREM/slow-wave sleep (raphe nuclei) |
| Stahl's / Modern neuroscience | Part of arousal/wakefulness ARAS |
| Both agree | Serotonin → melatonin (sleep-promoting indirectly) |
For First MBBS exams (based on Guyton): Serotonin from raphe nuclei → promotes sleep. Decreased serotonin → decreased sleep. Answer remains A. Serotonin, but the Guyton-based explanation is: "raphe nuclei release serotonin to promote slow-wave sleep."
Thank you for the correction - the modern understanding does place serotonin in the arousal system, and the exam answer depends on which textbook framework is being used. For Guyton-based MBBS exams, A. Serotonin is still the answer but via the raphe nucleus → slow-wave sleep pathway.
This is a shared conversation. Sign in to Orris to start your own chat.