Case Study Answers - Biochemistry, YIMS Yadgiri

CASE 1 (Page 12): 7-month-old male - Vomiting, Jaundice, Failure to Thrive

Q1. Probable Diagnosis

• Robbins & Kumar Basic Pathology confirms: galactose-1-phosphate accumulates in liver, spleen, lens, kidney, cerebral cortex, and red cells.

Q2. Biochemical Basis of Cataract

1. In the lens, excess galactose is reduced by aldose reductase to galactitol (also called dulcitol).
2. Galactitol cannot be further metabolized - it has no exit pathway from the lens cells.
3. It accumulates and creates a high osmotic gradient, drawing water into lens fibers.
4. This causes osmotic swelling and disruption of lens fiber architecture, leading to opacity (cataract).

1. Galactose is phosphorylated to galactose-1-phosphate by galactokinase.
2. Due to transferase deficiency, Gal-1-P cannot be converted onwards and accumulates in lens cells.
3. Gal-1-P is directly toxic to lens cells, disrupting protein structure and contributing to opacity.

Adams & Victor's Principles of Neurology confirms: "Galactosemia is a cause of cataract; the mechanism of cataract formation is the accumulation of dulcitol in the lens." Robbins & Kumar: "galactose-1-phosphate and other metabolites, including galactitol, accumulate in many tissues including...the lens of the eye."

Q3. Biochemical Basis of Hepatomegaly

1. Dietary lactose (breast milk) is digested to glucose + galactose.
2. Galactose is phosphorylated to galactose-1-phosphate by galactokinase.
3. Due to galactose-1-phosphate uridylyltransferase deficiency, Gal-1-P cannot be converted to glucose-1-phosphate and UDP-galactose.
4. Gal-1-P accumulates in hepatocytes - it is directly hepatotoxic.
5. This causes:
   • Fatty change (steatosis) in hepatocytes - the primary cause of early hepatomegaly
   • Hepatocellular damage and dysfunction
   • Progressive cirrhosis if untreated
6. Additionally, galactitol accumulation contributes to osmotic stress in liver cells.

Robbins & Kumar: "The early-onset hepatomegaly results largely from fatty change" due to Gal-1-P toxicity. Harrison's 22e: "galactose-1-phosphate accumulates and results in injury to kidneys, liver, CNS, eyes..." presenting with "vomiting, diarrhea, jaundice, hepatomegaly." Tintinalli's EM: "Galactosemia results from deficiency of galactose-1-phosphate uridylyltransferase, leading to accumulation of galactose-1-phosphate and other metabolites that are toxic to the liver."

CASE 2 (Page 13): 3-month-old male - Unconscious, Doll-like face

Q1. Probable Diagnosis

Q2. Deficient Enzyme

Basic Medical Biochemistry (Lippincott): "This child has glucose-6-phosphatase deficiency (GSD type Ia, von Gierke disease)." Goldman-Cecil Medicine: "Homozygous glucose-6-phosphatase deficiency (GSD type I, Von Gierke disease) is the most common glycogen storage disease that causes hypoglycemia."

Q3. Biochemical Basis / Glycogen Deposition (Hepatomegaly)

1. G6P cannot be dephosphorylated - free glucose cannot be released from the liver into the blood.
2. During fasting, despite normal glycogen breakdown (phosphorylase is functional), all glycogenolysis and gluconeogenesis produces G6P that cannot exit the cell.
3. G6P accumulates → drives glycogen synthase activity → massive glycogen synthesis and deposition in hepatocytes and renal tubular cells.
4. Simultaneously, G6P is shunted into glycolysis → pyruvate → lactate (causing lactic acidosis) and into the pentose phosphate pathway → excess NADPH → de novo lipogenesis → lipid (triglyceride) accumulation in hepatocytes.
5. Result: hepatocytes become massively distended with both glycogen AND lipid - explaining the liver biopsy findings directly.

Lippincott Biochemistry: "Liver biopsy - hepatocytes enlarged; staining reveals large amounts of lipid (primarily triacylglycerol) and carbohydrate. Liver glycogen elevated in amount and normal in structure. Enzyme assay: <10% of normal glucose-6-phosphatase activity."

Summary Comparison

Case Study Answers - Biochemistry, YIMS Yadgiri (Pages 13-14)

CASE 2 (Page 13) - FULL ANSWER: Von Gierke Disease

Q1. Probable Diagnosis

Q2. Biochemical Defect

Tietz Laboratory Medicine (7th ed): "Type Ia (von Gierke disease) is caused by a deficiency of the glucose-6-phosphatase catalytic subunit...Type Ia constitutes ~80% of GSD type I."

Q3. Biochemical Basis of Hepatomegaly AND Hyperuricemia

A) Biochemical Basis of Hepatomegaly

1. G6P cannot be converted to free glucose - it is trapped inside hepatocytes.
2. The accumulating G6P:
   • Drives glycogen synthase forward → massive glycogen synthesis and deposition (explains distended hepatocytes with glycogen on biopsy)
   • Is channeled into glycolysis → pyruvate → lactate (explains elevated serum lactate = 95 mg/dL)
   • Enters de novo lipogenesis (via excess acetyl-CoA and NADPH from the pentose phosphate pathway) → triglyceride and cholesterol synthesis (explains elevated TG = 300 mg/dL, cholesterol = 350 mg/dL, and lipid vacuoles on biopsy)
3. The net result is hepatocytes engorged with both glycogen AND lipid → massive hepatomegaly.

Tietz Laboratory Medicine (7th ed): "The disease is characterized by (1) massive hepatomegaly, (2) growth retardation, (3) fasting hypoglycemia, (4) increased lactic acid in the blood (caused by excessive glycolysis), (5) hyperuricemia, and (6) hypertriglyceridemia."

B) Biochemical Basis of Hyperuricemia (Serum Uric acid = 9.5 mg/dL)

1. Trapped G6P is shunted into the Pentose Phosphate Pathway (HMP shunt)
2. This generates excess ribose-5-phosphate
3. Ribose-5-P is used to synthesize purine nucleotides (AMP, GMP) via de novo purine synthesis
4. These purines are eventually catabolized → hypoxanthine → xanthine → uric acid (by xanthine oxidase)
5. Result: overproduction of uric acid

1. Blocked gluconeogenesis/glycogenolysis → G6P overflow into glycolysis → massive lactate production (serum lactate = 95 mg/dL in this case)
2. Lactate and urate compete for the same renal tubular secretory transporters (OAT transporters)
3. High lactate levels competitively inhibit renal tubular urate secretion - the kidney cannot excrete uric acid normally
4. Result: uric acid is retained in the blood → hyperuricemia

Tietz Laboratory Medicine (7th ed): "Hyperuricemia caused by competitive inhibition by lactate of renal tubular urate secretion AND increased uric acid production." Basic Medical Biochemistry (Lippincott): "The elevated lactate levels block the ability of the kidney to remove urate from the blood, leading to hyperuricemia and gout."

CASE 3 (Page 14): Child with Diarrhea after Dairy Products

Q1. Probable Diagnosis

• Symptoms triggered specifically by dairy products (which contain lactose)
• Stool positive for reducing sugar (undigested lactose is a reducing sugar that appears in stool)
• Dehydration from osmotic diarrhea

Harrison's Principles (22e): "Lactose intolerance due to lactase deficiency is the most common brush border disaccharidase deficiency and is a frequent cause of diarrhea, abdominal pain, gassiness, and bloating."

Q2. Biochemical Defect

1. Lactase is absent/deficient on the intestinal brush border
2. Ingested lactose passes undigested into the colon
3. Colonic bacteria ferment lactose → produce:
   • Short-chain fatty acids (lower colonic pH)
   • H₂, CO₂, methane gas → bloating and flatulence
   • Organic acids → osmotic load
4. Undigested lactose and its fermentation products create an osmotic gradient → water is drawn into the colon → osmotic diarrhea
5. Rapid transit and fermentation → vomiting and abdominal cramps
6. Stool contains unabsorbed lactose = positive reducing sugar test (Clinitest/Benedict's)
7. Dehydration results from fluid loss in stool

Yamada's Gastroenterology (7th ed): "Undigested lactose in the GI tract is fermented by bacteria in the distal ileum and colon to produce short-chain fatty acids, hydrogen, carbon dioxide, and methane. Thus, bloating, flatulence, and diarrhea with resulting abdominal discomfort are typical features." Lippincott Biochemistry (8th ed): "With insufficient lactase there is an inability to fully digest dairy products. After consuming dairy, lactose intolerant individuals can experience cramping, diarrhea, and bloating."

Quick Summary: All 3 Cases

Case Study Answers - Biochemistry, YIMS Yadgiri (Pages 18-19)

CASE 4 (Page 18) - Major Case (12 Marks): 60-year-old male

Q1. Probable Diagnosis

Robbins & Kumar Pathologic Basis of Disease: "The clinicopathologic diagnosis of multiple myeloma relies on identification of clonal plasma cells in the marrow and the presence of CRAB criteria (hypercalcemia, renal dysfunction, anemia, and bone lesions)."

• Serum globulins 5.9 gm/dL (markedly elevated)
• A/G ratio reversed: Albumin 3.0 / Globulin 5.9
• ESR 100 mm/hr (very high - due to rouleaux formation by M proteins)
• M-band (monoclonal spike) on serum electrophoresis

Q2. Biochemical Cause (the handwritten answer is incomplete - here is the full answer)

1. Normal plasma cells produce polyclonal immunoglobulins (diverse - seen as broad band on electrophoresis)
2. In myeloma, a single malignant plasma cell clone expands in the marrow and produces massive amounts of one identical immunoglobulin (monoclonal = M protein)
3. The M protein is most commonly IgG (~55%) or IgA (~25%)
4. This monoclonal Ig floods the serum → elevated total protein and globulin fraction
5. On serum protein electrophoresis (SPEP): appears as a sharp, narrow "M-spike" (not a broad band) in the gamma region - because all molecules are identical in size/charge

• Elevated ESR (100 mm/hr): M proteins coat red blood cells causing rouleaux formation (stacking like coins) → increased sedimentation rate
• Hypercalcemia (13.0 mg/dL): Myeloma cells secrete osteoclast-activating factors (IL-1β, IL-6, RANKL, TNF-α) → osteoclast-mediated bone resorption → calcium released into blood; also PTHrP production in some cases
• Anemia (Hb 7.0): Myeloma cells crowd out normal erythroid precursors in bone marrow; renal failure reduces EPO production; M proteins can coat RBCs
• Renal failure (Cr 2.0): Multiple mechanisms:
  • Cast nephropathy - free light chains (Bence Jones proteins) precipitate in renal tubules forming obstructing casts
  • Hypercalcemia → nephrocalcinosis, nephrogenic DI
  • Amyloid (AL) deposition in glomeruli
• Bone pain/lytic lesions: Osteoclast-activating cytokines destroy cortical bone → punched-out lytic lesions on X-ray (skull, vertebrae, pelvis) - note that ALP is NOT markedly elevated because osteoblast activity is suppressed (unlike metastatic bone disease)
• Low albumin (3.0 gm/dL): Malnutrition, chronic disease state; myeloma suppresses normal immunoglobulin synthesis → immunoparesis

Robbins: "In 99% of patients, laboratory analyses reveal increased levels of immunoglobulin in the blood and/or light chains (Bence Jones proteins) in the urine. Most myelomas are associated with more than 3 g/dL of serum immunoglobulin. The most common M protein is IgG (~55%), followed by IgA (~25%)."

Q3. What are Bence Jones Proteins?

1. In normal plasma cells, heavy chains and light chains are synthesized in balanced amounts and assembled into complete immunoglobulins.
2. In myeloma, light chain synthesis often exceeds heavy chain synthesis.
3. Excess free light chains (FLC) are not incorporated into complete immunoglobulins and are secreted into the blood.
4. Being small (~25 kDa for a monomer, ~50 kDa for a dimer), they are freely filtered by the glomerulus into the urine.
5. They appear in urine as Bence Jones proteins.

• Present in 50-80% of multiple myeloma patients in urine
• The classic heat test: Bence Jones proteins precipitate at 40-60°C and redissolve at 100°C (heat-labile proteinuria) - though now replaced by immunofixation electrophoresis
• Detected reliably by urine immunofixation electrophoresis (IFE)
• In ~20% of myeloma patients, only free light chains are present (light chain myeloma) with no serum M protein

• Free light chains are directly nephrotoxic - they precipitate in distal tubules with Tamm-Horsfall protein → form casts → tubular obstruction → "myeloma kidney" (cast nephropathy)
• Can also deposit as amyloid fibrils (AL amyloidosis) in multiple organs

Tietz Laboratory Medicine: "In multiple myeloma, complete monoclonal Igs (usually IgG or IgA) are accompanied by variable concentrations of free light chains (FLCs) that appear in the urine as Bence Jones proteins (named after Henry Bence Jones who first described these in 1848)." Henry's Clinical Diagnosis: "Bence Jones protein is the light chain from a monoclonal immunoglobulin that is filtered in the kidney. The incidence of Bence Jones proteinuria in multiple myeloma has been estimated as 50-80%."

CASE 5 (Page 19) - Major Case (12 Marks): 42-year-old female

Q1. Probable Diagnosis

Lippincott Biochemistry (8th ed): "Abdominal obesity is associated with a cluster of metabolic abnormalities (hyperglycemia, insulin resistance, hyperinsulinemia, atherogenic dyslipidemia [high LDL, low HDL, elevated TAG], and hypertension) that is referred to as metabolic syndrome. It is a risk factor for developing CVD and T2D."

Q2. Biochemical Defect

1. Visceral/abdominal adiposity (waist 93 cm) → adipocytes become enlarged and dysfunctional
2. Visceral fat releases excess free fatty acids (FFAs) into portal circulation + secretes pro-inflammatory cytokines (IL-6, TNF-α) and reduces adiponectin (an insulin sensitizer)
3. TNF-α and FFAs impair insulin receptor signaling (by serine phosphorylation of IRS-1, blocking normal tyrosine phosphorylation) → insulin resistance in muscle, liver, and adipose tissue
4. The pancreatic β-cells compensate by secreting more insulin → hyperinsulinemia
5. Over time, β-cell exhaustion → impaired glucose tolerance → Type 2 diabetes

• Hyperglycemia (FPG 130 mg/dL): Insulin-resistant liver continues unchecked gluconeogenesis; skeletal muscle fails to take up glucose; impaired GLUT-4 translocation
• Hypertriglyceridemia (TG 250 mg/dL): Insulin resistance → uninhibited hormone-sensitive lipase in adipose → excess FFA to liver → de novo lipogenesis → excess VLDL-TG secretion; also insulin normally activates LPL (which clears TG) - resistance blocks this
• Low HDL (40 mg/dL): Elevated TG activates CETP (cholesteryl ester transfer protein) → exchanges TG for cholesterol esters in HDL → HDL becomes TG-rich → degraded faster by hepatic lipase → HDL falls
• Hypertension (160/100 mmHg): Hyperinsulinemia → increased renal sodium retention (via Na⁺/K⁺-ATPase) → volume expansion; activates sympathetic nervous system; stimulates endothelin-1 and reduces nitric oxide → vasoconstriction
• Breathlessness + palpitations: Consequence of hypertension leading to left ventricular hypertrophy/strain → reduced cardiac reserve

Lippincott Biochemistry: "The low-grade, chronic, systemic inflammation seen with obesity contributes to the pathogenesis of insulin resistance and T2D. In obesity, adipocytes release proinflammatory mediators such as IL-6 and TNF-α. Additionally, levels of adiponectin, which normally dampens inflammation and sensitizes tissues to insulin, are low."

Q3. (Expected: Complications / Treatment / Why HDL is low / Biochemical basis of hypertension)

• Weight loss → reduces FFA flux, restores insulin sensitivity, lowers TG, raises HDL
• Exercise → increases GLUT-4 in muscle, improves insulin signaling
• Metformin → reduces hepatic gluconeogenesis (activates AMPK)
• Statins → reduce VLDL/LDL; fibrates → activate PPARα → increase LPL activity → reduce TG

Summary of Both Cases

Case Study Answers - Biochemistry, YIMS Yadgiri (Pages 19-20)

CASE 4 (Page 19) - Multiple Myeloma: Handwritten Answer Review

• Q1 (Diagnosis): Multiple Myeloma - correct
• Q2 (Biochemical cause): "Drastic increase in globulins seen in paraproteinemias when a sharp spike is noticed in electrophoresis - M band" - correct but incomplete (full answer provided previously)
• Q3 (Bence Jones proteins): "Monoclonal light chains produced by plasmacytomas (plasma cells)" - correct

CASE 5 (Pages 19-20) - Major Case (12 Marks): Metabolic Syndrome

• Q1 → Metabolic Syndrome ✓
• Q2 → Waist circumference >80 cm ✓
• Q3 → Hyperlipidemia, hyperinsulinemia, hypertension, impaired glucose tolerance ✓

Q1. Probable Diagnosis

Q2. Criteria for Women (IDF/ATP-III Diagnostic Criteria)

• Waist cutoff for women = ≥80 cm (vs. ≥94 cm for men in IDF; or >88 cm for women in ATP-III)
• HDL cutoff for women = <50 mg/dL (vs. <40 mg/dL for men) - this patient's HDL of 40 mg/dL is well below even the male cutoff

Q3. Two Complications of Metabolic Syndrome

• Atherosclerosis - insulin resistance → endothelial dysfunction → oxidized LDL accumulation in vessel walls → atherosclerotic plaques
• Coronary Artery Disease (CAD) - myocardial infarction
• Stroke - cerebrovascular disease
• Left Ventricular Hypertrophy - as in this patient (breathlessness + palpitations from sustained hypertension)

• Chronic insulin resistance → pancreatic β-cells must secrete more insulin (hyperinsulinemia)
• Over time, β-cell exhaustion occurs → insulin secretion fails
• Fasting glucose 130 mg/dL in this patient is already in the diabetic range (≥126 mg/dL) - she likely already has T2DM

Lippincott Biochemistry (8th ed): "Metabolic syndrome is a risk factor for developing CVD and T2D. The low-grade, chronic, systemic inflammation seen with obesity contributes to the pathogenesis of insulin resistance and T2D." Campbell Walsh Wein Urology: "Metabolic syndrome is defined as a series of interconnected biochemical, physiologic, metabolic, and clinical factors that increase the individual's risk of type 2 diabetes mellitus, heart disease, and early mortality."

Regarding the New Case (Page 20 - Bottom)

Summary Table - Metabolic Syndrome Criteria (Women)