Clinical Biochemistry Practice Questions

Q: In C-reactive protein (CRP), the 'C' stands for which of the following?

C polysaccharide of Streptococcus. ### Explanation **Correct Answer: D. C polysaccharide of Streptococcus** **Why it is correct:** C-reactive protein (CRP) was first discovered in 1930 by Tillet and Francis. It was named so because of its unique ability to precipitate the **C-polysaccharide (a somatic antigen) of *Streptococcus pneumoniae*** in the presence of calcium. It is a classic acute-phase reactant synthesized by the liver in response to interleukin-6 (IL-6). **Analysis of Incorrect Options:** * **A. Canavallin A:** This is a lectin (carbohydrate-binding protein) derived from the jack bean. It is used in research to study cell surface glycoproteins but has no relation to the naming of CRP. * **B. Cellular:** While CRP is part of the innate immune response, the 'C' does not stand for cellular. CRP is a secreted plasma protein, not a cellular component. * **C. Chondroitin sulfate:** This is a glycosaminoglycan found in connective tissues and cartilage. It does not react with CRP. **High-Yield Clinical Pearls for NEET-PG:** * **Structure:** CRP is a member of the **pentraxin** family, consisting of five identical non-covalently bound subunits. * **Function:** It acts as an **opsonin**, activating the classical complement pathway by binding to phosphocholine expressed on the surface of dead cells and bacteria. * **Clinical Marker:** It is a non-specific marker of inflammation. **hs-CRP (high-sensitivity CRP)** is specifically used as a biomarker to assess the risk of cardiovascular disease (Atherosclerosis). * **Kinetics:** CRP levels rise rapidly (within 6–12 hours) and have a short half-life (19 hours), making it an excellent marker for monitoring disease activity and response to treatment.

Q: Which of the following is used for testing absorption in the intestine?

Xylose. **Explanation:** The **D-Xylose absorption test** is a classic clinical tool used to differentiate between **malabsorption** (mucosal disease) and **maldigestion** (pancreatic insufficiency). **Why Xylose is correct:** D-Xylose is an eight-carbon aldopentose that is absorbed via passive diffusion in the proximal small intestine. Unlike other sugars, it does not require pancreatic enzymes (like amylase) or bile salts for digestion. Once absorbed, it is not metabolized by the liver and is excreted unchanged in the urine. Therefore, low levels of xylose in the blood or urine after oral administration indicate a **mucosal defect** in the small intestine (e.g., Celiac disease, Tropical sprue, or Whipple’s disease). **Why the other options are incorrect:** * **Sucrose, Maltose, and Fructose:** These are common dietary sugars. Sucrose and Maltose are disaccharides that require specific brush-border enzymes (**Sucrase** and **Maltase**) for breakdown into monosaccharides before absorption. Fructose is a monosaccharide but is absorbed via facilitated diffusion (GLUT-5). Because these sugars are rapidly metabolized by the body for energy, they cannot be used as reliable markers to quantify intestinal absorptive capacity in a standardized test. **High-Yield Clinical Pearls for NEET-PG:** * **Normal Test Result:** High urinary excretion of xylose (>4g in 5 hours) suggests the intestinal mucosa is intact, pointing toward **Pancreatic Insufficiency** as the cause of steatorrhea. * **Abnormal Test Result:** Low urinary excretion suggests **Intestinal Mucosal Disease**. * **False Positives:** Low urinary xylose can occur in patients with **renal dysfunction**, ascites, or Small Intestinal Bacterial Overgrowth (SIBO), as bacteria may metabolize the xylose before it is absorbed.

Q: Glycosylated hemoglobin reflects the mean blood glucose level of the previous?

3 months. **Explanation:** **1. Why 3 months is correct:** Glycosylated hemoglobin (HbA1c) is formed by the **non-enzymatic glycation** of the N-terminal valine of the beta chain of hemoglobin. This process is irreversible and occurs at a rate proportional to the blood glucose concentration. Because hemoglobin resides within red blood cells (RBCs), the HbA1c level reflects the average glucose concentration over the **lifespan of the RBC**, which is approximately **120 days (roughly 3–4 months)**. However, because newer RBCs contribute more to the total hemoglobin pool than older ones, the value most accurately reflects the preceding **8–12 weeks**. **2. Why other options are incorrect:** * **15 days:** This is too short. While 15-day trends can be measured by **Fructosamine** (glycosylated albumin), which has a half-life of ~2-3 weeks, HbA1c remains stable for much longer. * **1 month:** This only captures the most recent portion of the RBC lifespan and does not represent the full diagnostic window of HbA1c. * **6 months:** This exceeds the average 120-day survival of an erythrocyte; by 6 months, the original population of RBCs has been replaced. **3. High-Yield Clinical Pearls for NEET-PG:** * **Diagnostic Cut-off:** According to ADA criteria, HbA1c **≥ 6.5%** is diagnostic for Diabetes Mellitus. * **Falsely Low HbA1c:** Seen in conditions with increased RBC turnover (e.g., Hemolytic anemia, recent blood transfusion, or pregnancy). * **Falsely High HbA1c:** Seen in conditions that prolong RBC lifespan (e.g., Vitamin B12 or Folate deficiency anemia) or Splenectomy. * **Formula:** Estimated Average Glucose (eAG) in mg/dL = $(28.7 \times HbA1c) - 46.7$.

Q: What is used to assess the long-term control of diabetes?

HbA1c. **Explanation:** **HbA1c (Glycated Hemoglobin)** is the gold standard for assessing long-term glycemic control. It is formed by the non-enzymatic, irreversible attachment of glucose to the N-terminal valine of the beta chain of hemoglobin (Glycation). Since the average lifespan of a Red Blood Cell (RBC) is **120 days**, the HbA1c level reflects the average blood glucose concentration over the preceding **2–3 months**. **Analysis of Incorrect Options:** * **A. Random Blood Sugar:** This provides a "snapshot" of the blood glucose at a single point in time. It is influenced by recent meals or physical activity and cannot reflect long-term control. * **B. Microalbuminuria/creatinine ratio:** This is a screening tool for **Diabetic Nephropathy** (early kidney damage), not a measure of glycemic control. * **C. HbA2:** This is a minor variant of adult hemoglobin ($\alpha_2\delta_2$). Elevated levels are diagnostic for **Beta-thalassemia trait**, having no clinical relevance to diabetes monitoring. **High-Yield Clinical Pearls for NEET-PG:** * **Target Level:** For most diabetic patients, the goal is an HbA1c **< 7%**. * **Fructosamine Test:** Measures glycated albumin and reflects glucose control over the past **2–3 weeks**. It is used when HbA1c is unreliable (e.g., Hemolytic anemia, Pregnancy, or Sickle cell disease). * **False Low HbA1c:** Seen in conditions that decrease RBC lifespan (Hemolysis, recent blood transfusion, or Erythropoietin therapy). * **False High HbA1c:** Seen in Iron deficiency anemia (due to increased RBC lifespan).

Q: Which myocardial isozyme of CK is specific for myocardial infarction?

CK-MB. **Explanation:** Creatine Kinase (CK) is a dimeric enzyme composed of two subunits: **M (Muscle)** and **B (Brain)**. These subunits combine to form three distinct isoenzymes, each with specific tissue distributions. **Why CK-MB is the Correct Answer:** **CK-MB (MB variant)** is primarily found in the **myocardium** (cardiac muscle). While a small amount exists in skeletal muscle, it makes up about 15-40% of the total CK in the heart. Following a Myocardial Infarction (MI), damaged cardiac myocytes leak CK-MB into the bloodstream. It typically rises within 4–6 hours, peaks at 12–24 hours, and returns to baseline within 48–72 hours. Its rapid clearance makes it particularly useful for detecting **re-infarction**. **Analysis of Incorrect Options:** * **CK-BB (Option A):** Found predominantly in the **Brain** and smooth muscle (e.g., lungs, bladder). It is rarely elevated in serum because it does not cross the blood-brain barrier easily. * **CK-MM (Option B):** Found primarily in **Skeletal Muscle** (99%) and the heart (60-80%). Elevations are seen in muscular dystrophy, strenuous exercise, or trauma, making it non-specific for cardiac injury. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard:** While CK-MB is the specific *isozyme* of CK, **Cardiac Troponins (I and T)** are currently the overall "Gold Standard" biomarkers for MI due to higher sensitivity and specificity. * **CK-MB Index:** If the CK-MB/Total CK ratio is **>5%**, it strongly suggests myocardial origin rather than skeletal muscle damage. * **Macro-CK:** An atypical form of CK (CK bound to IgG) that can cause a false elevation in total CK levels.

Question 1

In C-reactive protein (CRP), the 'C' stands for which of the following?

Accepted Answer

C polysaccharide of Streptococcus

Answer

Canavallin A

Answer

Cellular

Answer

Chondroitin sulfate

Question 2

Alcohol/ethanol induced fatty liver is caused by an increased ratio of?

Accepted Answer

NADH to NAD+

Answer

NAD+ to NADH

Answer

NADPH to NADP+

Answer

NADP+ to NADPH

Question 3

Which enzymes are normally found in cerebrospinal fluid?

Accepted Answer

CK and LDH

Answer

GGT and ALP

Answer

ALP and CK-MB

Answer

Deaminase and peroxidase

Question 4

Which of the following is NOT a cause of unconjugated hyperbilirubinemia?

Accepted Answer

Rotor syndrome

Answer

Hemolytic anemia

Answer

Large hematoma

Answer

Megaloblastic anemia

Question 5

A patient presents with jaundice and white stools. Which of the following enzymes is typically not elevated in this clinical scenario?

Accepted Answer

Glutamate reductase

Answer

5' nucleotidase

Answer

Alkaline phosphatase

Answer

Gamma glutamyl transpeptidase

Question 6

Which of the following is used for testing absorption in the intestine?

Accepted Answer

Xylose

Answer

Sucrose

Answer

Fructose

Answer

Maltose

Question 7

Glycosylated hemoglobin reflects the mean blood glucose level of the previous?

Accepted Answer

3 months

Answer

15 days

Answer

1 month

Answer

6 months

Question 8

What is the minimum level of TSH that can be detected by third-generation TSH assays?

Accepted Answer

0.04 IU/ml

Answer

0.4 IU/ml

Answer

0.004 IU/ml

Answer

4 IU/ml

Question 9

What is used to assess the long-term control of diabetes?

Accepted Answer

HbA1c

Answer

Random blood sugar

Answer

Microalbuminuria/creatinine ratio

Answer

HbA2

Question 10

Which myocardial isozyme of CK is specific for myocardial infarction?

Accepted Answer

CK-MB

Answer

CK-BB

Answer

CK-MM

Answer

All of the above

Clinical Biochemistry — MCQs

Clinical Biochemistry — MCQs

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