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Here are complete model answers for all questions in your Narayana Medical College - 1st MBBS Biochemistry Paper II (3rd Internal, 18-07-2025):
ESSAY QUESTION (10 marks)
Q1. Acid-Base Imbalance Case
Given values:
| Parameter | Patient Value | Normal |
|---|---|---|
| pH | 7.1 (↓) | 7.35-7.45 |
| Na+ | 146 meq/L | 135-145 |
| K+ | 5.5 meq/L (↑) | 3.5-5.0 |
| HCO3- | 21 meq/L (normal/slightly ↓) | 22-26 |
| pCO2 | 46 mm Hg (↑) | 35-45 |
Parameters Altered:
- pH is decreased (7.1) - severe acidemia
- pCO2 is elevated (46 mm Hg) - above normal upper limit of 45
- K+ is elevated (5.5) - hyperkalemia
- HCO3- is at low-normal (21 meq/L) - not adequately compensating
Probable Diagnosis: Acute Respiratory Acidosis
Reasoning:
- Primary defect: pCO2 is elevated due to rib fracture + compromised breathing causing hypoventilation (CO2 cannot be exhaled)
- CO2 + H2O → H2CO3 → H+ + HCO3- (pH falls)
- The HCO3- is only 21 (not elevated), indicating acute respiratory acidosis with inadequate metabolic compensation - kidneys have not yet had time to retain HCO3-
- Hyperkalemia (K+ 5.5) occurs because: as cells buffer H+ by taking it in, K+ exits the cells in exchange (for every 0.1 pH unit fall, K+ rises ~0.6 mmol/L)
Henderson-Hasselbalch equation: pH = pK + log [HCO3-] / [CO2] = 6.1 + log 21/1.38 = ~7.3 (consistent with respiratory acidosis)
Note on pH Imbalance
Definition: pH imbalance occurs when arterial pH falls below 7.35 (acidosis) or rises above 7.45 (alkalosis).
Four primary acid-base disorders:
| Disorder | Primary defect | pH | pCO2 | HCO3- |
|---|---|---|---|---|
| Respiratory Acidosis | ↑ pCO2 (hypoventilation) | ↓ | ↑ | ↑ (compensation) |
| Respiratory Alkalosis | ↓ pCO2 (hyperventilation) | ↑ | ↓ | ↓ (compensation) |
| Metabolic Acidosis | ↓ HCO3- (acid gain/base loss) | ↓ | ↓ (compensation) | ↓ |
| Metabolic Alkalosis | ↑ HCO3- (acid loss/base gain) | ↑ | ↑ (compensation) | ↑ |
Clinical entities of Respiratory Acidosis:
- COPD, asthma, pneumonia
- Rib fracture / chest trauma (as in this case)
- Neuromuscular disorders (Guillain-Barre)
- Central respiratory depression (opioids, head injury)
- Pickwickian syndrome, sleep apnea
Lines of Defence (Buffer Systems)
1st Line - Chemical Buffers (Immediate, within seconds):
a) Bicarbonate-Carbonic Acid Buffer (most important in ECF):
- CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3-
- pKa = 6.1; works best at physiological pH due to open system (lungs regulate CO2)
b) Haemoglobin Buffer (most important in blood):
- H+ + Hb- → HHb (deoxyhaemoglobin has greater buffering capacity)
- Isohydric shift: CO2 diffuses into RBC, is hydrated by carbonic anhydrase to H2CO3, releasing H+ which is buffered by Hb
c) Phosphate Buffer:
- H2PO4- / HPO4-2 (pKa 6.8) - important in urine and ICF
d) Protein Buffers:
- Plasma proteins (albumin) - histidine residues act as H+ acceptors
2nd Line - Respiratory Compensation (minutes to hours):
- In metabolic acidosis: ↓pH stimulates the medullary chemoreceptors → hyperventilation (Kussmaul breathing) → ↓pCO2 → pH rises toward normal
- Compensation formula: Expected pCO2 = (1.5 × HCO3-) + 8 ± 2 (Winter's formula)
- In respiratory acidosis: increased rate and depth of respiration eliminate CO2 (if the respiratory centre is intact)
3rd Line - Renal Compensation (hours to days, most powerful):
The kidneys restore acid-base balance by:
- Bicarbonate reabsorption: In proximal tubule - HCO3- is reclaimed via Na+-H+ antiporter and carbonic anhydrase
- Titratable acid excretion: H+ combines with HPO4-2 in tubular fluid to form H2PO4- which is excreted
- Ammoniagenesis: Glutamine → NH3 (glutaminase) → NH3 + H+ → NH4+ (excreted in urine)
- This is the dominant mechanism in chronic acid loads
In chronic respiratory acidosis: kidneys retain HCO3- to compensate - for every 10 mm Hg rise in pCO2, HCO3- increases ~3.5 mmol/L (chronic) vs ~1 mmol/L (acute). (Tietz Textbook of Laboratory Medicine, 7th Ed.)
SHORT ESSAY QUESTIONS (5 marks each)
Q2. Hormone Receptors Classification + Insulin Mechanism of Action
Classification of Hormone Receptors
Type I - Cell Surface (Membrane) Receptors: For water-soluble hormones (peptides, catecholamines) that cannot cross the plasma membrane.
| Subtype | Mechanism | Examples |
|---|---|---|
| G protein-coupled receptors (GPCR) | 7 transmembrane domains; activate Gs/Gi/Gq proteins | Glucagon, TSH, LH, FSH, PTH, ADH (V2), Adrenaline (β-receptors) |
| Receptor Tyrosine Kinase (RTK) | Intrinsic tyrosine kinase activity; autophosphorylation | Insulin, IGF-1, EGF, PDGF, FGF |
| JAK-STAT receptors | Non-intrinsic tyrosine kinase; signal via JAK2 | Growth hormone, Prolactin, Cytokines |
| Receptor Serine/Threonine Kinase | Intrinsic Ser/Thr kinase | TGF-β |
| Guanylyl Cyclase Receptors | Generate cGMP as second messenger | ANP (Atrial Natriuretic Peptide) |
| Ion channel-linked (Ionotropic) | Direct ion channel gating | Acetylcholine (nicotinic), GABA |
Type II - Intracellular (Nuclear) Receptors: For lipid-soluble hormones that cross the plasma membrane.
| Subtype | Location | Examples |
|---|---|---|
| Nuclear receptors (Type I) | Cytoplasm → nucleus | Steroid hormones (cortisol, aldosterone, testosterone, estrogen) |
| Nuclear receptors (Type II) | Nucleus | Thyroid hormones (T3/T4), Vitamin D, Retinoic acid |
Mechanism of Action of Insulin (Signal Transduction)
Insulin receptor is a Receptor Tyrosine Kinase (RTK).
Structure: Heterotetrameric glycoprotein - 2α + 2β subunits linked by disulfide bonds. α-subunits are extracellular (insulin binding site); β-subunits are transmembrane with intrinsic tyrosine kinase domain.
Steps:
- Insulin binding: Insulin binds to the α-subunit of its receptor
- Conformational change: Binding causes a conformational change transmitted to β-subunits
- Autophosphorylation: β-subunit tyrosine kinase is activated → phosphorylates tyrosine residues on the other β-subunit (transphosphorylation)
- IRS phosphorylation: Activated receptor phosphorylates Insulin Receptor Substrate (IRS-1, IRS-2) proteins on tyrosine residues
- Two major downstream pathways:
- PI3K-Akt pathway: Phosphorylated IRS activates PI3-kinase (Phosphatidylinositol-3-kinase) → generates PIP3 → activates PDK1 → activates Akt (PKB) → stimulates GLUT-4 translocation to membrane (glucose uptake in muscle/adipose), inhibits glycogen synthase kinase-3 (promoting glycogen synthesis), inhibits FOXO (promoting survival)
- MAPK/Ras pathway: IRS activates Grb2/SOS → activates Ras → Raf → MEK → ERK (MAPK) → nucleus → gene transcription (growth and proliferation effects)
Metabolic effects of insulin signaling:
- ↑ GLUT-4 translocation → ↑ glucose uptake (muscle, fat)
- ↑ Glycogen synthesis (activates glycogen synthase)
- ↑ Protein synthesis
- ↑ Lipogenesis, ↓ lipolysis
- ↓ Gluconeogenesis and glycogenolysis (in liver)
Q3. Autoimmunity + Functions of Immunoglobulins
Autoimmunity
Definition: Autoimmunity is a state in which the immune system fails to maintain self-tolerance and mounts an immune response against the body's own (self) antigens (autoantigens), resulting in tissue damage.
Mechanisms of loss of tolerance:
- Failure of clonal deletion in thymus (central tolerance failure)
- Molecular mimicry (microbial antigens resembling self-antigens)
- Bystander activation (inflammation reveals hidden self-antigens)
- Polyclonal B/T cell activation
- Defective Treg (regulatory T cell) function
Examples: Hashimoto thyroiditis, Graves' disease, SLE, Rheumatoid arthritis, Type 1 diabetes mellitus, Myasthenia gravis
Functions of Immunoglobulins (Antibodies)
Immunoglobulins are glycoproteins produced by plasma cells (differentiated B lymphocytes). All share a basic Y-shaped structure: 2 heavy + 2 light chains with variable (Fab) and constant (Fc) regions.
| Ig Class | Key Properties & Functions |
|---|---|
| IgG (most abundant, ~75%) | - Only Ig that crosses the placenta (passive immunity to fetus) - Opsonization (coats bacteria for phagocytosis) - Activates complement (classical pathway) - Neutralization of toxins and viruses - Long-term secondary immune response - 4 subclasses (IgG1-4) |
| IgA (secretory) | - Present in secretions: saliva, tears, breast milk, intestinal mucus, respiratory secretions - First line of mucosal defense - Exists as monomer (serum) and dimer (secretory, with J-chain and secretory component) - Neutralizes pathogens at mucosal surfaces - Does NOT activate complement - Passed to infant in breast milk (passive mucosal immunity) |
| IgM (largest, pentamer) | - First antibody produced in primary immune response - Most efficient activator of classical complement pathway - Agglutination (5 binding sites as pentamer) - Acts as B-cell antigen receptor (monomer) - Natural antibodies (ABO blood group) |
| IgE (lowest serum concentration) | - Binds to Fc receptors on mast cells and basophils - Triggers Type I hypersensitivity (allergy, anaphylaxis) - Role in defense against parasites (helminths) - Mediates release of histamine, prostaglandins on re-exposure to antigen |
| IgD | - Present mainly on surface of naive B lymphocytes (as B-cell receptor) - Role in B-cell maturation and activation - Very low serum concentration - Exact function not fully understood |
Q4. Hypothyroidism Case (58-year-old female)
Given: Weight gain, cold intolerance, sleep disturbance, menstrual irregularities
- Total T4: 3.5 µg/dl (↓ low)
- TSH: 19 µU/ml (↑ high)
- Serum cholesterol: 300 mg/dl (↑ high)
A) Probable Diagnosis: Primary Hypothyroidism (most likely Hashimoto's thyroiditis)
Reasoning: Low T4 + elevated TSH = primary thyroid gland failure. The pituitary is compensating by secreting excess TSH to stimulate a failing thyroid. The combination of symptoms (weight gain, cold intolerance, menstrual disturbance) and hypercholesterolemia are classic. (Robbins & Kumar Basic Pathology)
B) Normal Reference Ranges
| Hormone | Normal Range |
|---|---|
| Total T3 | 80-200 ng/dl (1.2-3.1 nmol/L) |
| Total T4 | 5-12 µg/dl (64-154 nmol/L) |
| TSH | 0.4-4.0 µIU/ml |
This patient has T4 of 3.5 (low) and TSH of 19 (high) - confirms primary hypothyroidism.
C) How Cholesterol is Affected in Hypothyroidism
Thyroid hormones normally upregulate LDL receptors on hepatocytes and stimulate the activity of cholesterol 7α-hydroxylase (rate-limiting enzyme in bile acid synthesis - the main pathway of cholesterol excretion).
In hypothyroidism:
- ↓ T3/T4 → ↓ LDL receptors → reduced hepatic clearance of LDL from blood
- ↓ Activity of cholesterol 7α-hydroxylase → reduced conversion of cholesterol to bile acids
- ↓ Lipoprotein lipase activity
- Net result: Hypercholesterolemia (elevated LDL, total cholesterol) - hence cholesterol of 300 mg/dl in this patient
- This increases cardiovascular risk
D) Foods to Avoid
The patient should avoid goitrogenic foods that further suppress thyroid function:
- Cruciferous vegetables (in excess): Cabbage, cauliflower, broccoli, Brussels sprouts, kale - contain isothiocyanates that inhibit iodine uptake by thyroid
- Soy products (in large amounts) - isoflavones can interfere with thyroid hormone synthesis and absorption of levothyroxine
- Cassava / millet - contain cyanogenic glycosides (goitrogens)
- Processed/high-fat foods - worsen hypercholesterolemia
- Should also avoid taking thyroid medication with calcium, iron, or soy (reduce absorption of levothyroxine)
E) Two Functions of Thyroid Hormones (T3/T4)
-
Regulation of Basal Metabolic Rate (BMR): Thyroid hormones increase the expression of Na+/K+ ATPase in cell membranes, increasing O2 consumption and heat production in virtually all tissues. They stimulate carbohydrate and lipid catabolism and protein synthesis - the net result is an increase in BMR. (Robbins & Kumar Basic Pathology)
-
CNS development and growth: T3 and T4 are essential for normal brain development, myelination of nerve fibres, and bone maturation in the fetus and infant. Deficiency causes cretinism (severe mental disability, short stature). In adults, they maintain normal cognitive function - hypothyroidism causes slowing of mental processes, depression, and myxoedema.
Diagrams to draw (examiner expects these):
- For Q1: Henderson-Hasselbalch equation, buffer system diagram, bicarbonate reabsorption in renal tubule
- For Q2: Insulin receptor structure (α2β2 tetramer), RTK signaling cascade with PI3K-Akt pathway
- For Q4: HPT axis (Hypothalamus → TRH → Pituitary → TSH → Thyroid → T3/T4) with negative feedback; cholesterol metabolism pathway
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