All Free Biology MCQs with Answers

861. The QRS complex in ECG represents

• A. Atrial depolarization
• B. Ventricular depolarization
• C. Atrial repolarization
• D. Ventricular repolarization

Explanation: The QRS complex represents ventricular depolarization (contraction); the P wave is atrial depolarization.

862. Which WBC is most abundant?

• A. Lymphocytes
• B. Monocytes
• C. Neutrophils
• D. Eosinophils

Explanation: Neutrophils are the most abundant WBCs (60-70%), functioning as primary phagocytes against bacteria.

863. Tissue fluid is formed by

• A. Active transport
• B. Ultrafiltration from capillaries
• C. Secretion by cells
• D. Osmosis only

Explanation: Tissue fluid forms by ultrafiltration from arterial ends of capillaries due to hydrostatic pressure.

864. Cardiac output equals

• A. Heart rate + Stroke volume
• B. Heart rate × Stroke volume
• C. Heart rate - Stroke volume
• D. Stroke volume only

Explanation: Cardiac output (mL/min) = Heart rate (beats/min) × Stroke volume (mL/beat).

865. Which is NOT a function of lymphatic system?

• A. Return tissue fluid to blood
• B. Fat absorption
• C. Immune defense
• D. Oxygen transport

Explanation: Oxygen transport is a function of blood, not lymph. Lymph returns tissue fluid, absorbs fats, and contains immune cells.

866. Fibrinogen is converted to fibrin by

• A. Prothrombin
• B. Thrombin
• C. Calcium
• D. Platelets

Explanation: Thrombin converts soluble fibrinogen to insoluble fibrin, forming the mesh of blood clots.

867. The resting membrane potential is approximately

• A. +70 mV
• B. -70 mV
• C. 0 mV
• D. -90 mV

Explanation: The resting membrane potential is about -70 mV (inside negative relative to outside), maintained by Na+/K+ pump and K+ leak channels.

868. Action potentials are conducted in one direction because

• A. The axon is myelinated
• B. Of the refractory period
• C. Of the all-or-none law
• D. Sodium channels are everywhere

Explanation: The refractory period (when Na+ channels are inactivated) prevents backward conduction, ensuring unidirectional propagation.

869. Myelination increases conduction velocity by

• A. Increasing axon diameter
• B. Saltatory conduction
• C. Increasing Na+ channels
• D. Decreasing resistance

Explanation: Myelin insulates the axon, allowing action potentials to jump between nodes of Ranvier (saltatory conduction), greatly increasing speed.

870. Neurotransmitters are released by

• A. Diffusion
• B. Exocytosis
• C. Endocytosis
• D. Osmosis

Explanation: Neurotransmitters are released by exocytosis of synaptic vesicles when Ca2+ enters the presynaptic terminal.