USMLE Step 1 Physiology Practice Questions

During strenuous exercise, muscle blood flow increases significantly. Which metabolic factor plays the most crucial role in causing local vasodilation in active skeletal muscle?

• (A) Adenosine
• (B) Potassium ions
• (C) Lactic acid
• (D) Carbon dioxide

What is the primary mechanism by which loop diuretics, such as furosemide, increase sodium excretion?

• (A) Inhibition of the Na+-K+-2Cl- cotransporter in the thick ascending limb
• (B) Inhibition of the Na+-Cl- cotransporter in the distal convoluted tubule
• (C) Blockade of epithelial sodium channels (ENaC) in the collecting duct
• (D) Antagonism of aldosterone receptors

In response to a decrease in arterial blood pressure, the baroreceptor reflex leads to which of the following physiological changes?

• (A) Increased heart rate and increased systemic vascular resistance
• (B) Decreased heart rate and decreased systemic vascular resistance
• (C) Increased heart rate and decreased systemic vascular resistance
• (D) Decreased heart rate and increased systemic vascular resistance

Which hormone, released from the duodenum in response to fatty acids and amino acids in chyme, stimulates gallbladder contraction and pancreatic enzyme secretion?

• (A) Cholecystokinin (CCK)
• (B) Secretin
• (C) Gastrin
• (D) Ghrelin

During the depolarization phase (phase 0) of the ventricular action potential, which ion channel is primarily responsible for the rapid influx of positive charge?

• (A) Voltage-gated sodium channels
• (B) Voltage-gated calcium channels (L-type)
• (C) Voltage-gated potassium channels (delayed rectifier)
• (D) Funny channels (If)

What is the physiological significance of the Bohr effect regarding oxygen transport?

• (A) Decreased pH shifts the oxygen-hemoglobin dissociation curve to the right, enhancing oxygen unloading in tissues
• (B) Increased pH shifts the oxygen-hemoglobin dissociation curve to the right, enhancing oxygen unloading in tissues
• (C) Decreased temperature shifts the oxygen-hemoglobin dissociation curve to the left, enhancing oxygen loading in the lungs
• (D) Increased 2,3-BPG shifts the oxygen-hemoglobin dissociation curve to the left, enhancing oxygen loading in the lungs

Glomerular filtration rate (GFR) is primarily determined by the balance of hydrostatic and oncotic pressures across the glomerular capillaries. Which pressure most directly opposes filtration?

• (A) Hydrostatic pressure in Bowman's capsule
• (B) Glomerular capillary hydrostatic pressure
• (C) Glomerular capillary oncotic pressure
• (D) Oncotic pressure in Bowman's capsule

Which substance is freely filtered at the glomerulus but is almost entirely reabsorbed in the proximal convoluted tubule under normal physiological conditions?

• (A) Glucose
• (B) Inulin
• (C) Creatinine
• (D) Para-aminohippuric acid (PAH)

Surfactant, produced by Type II pneumocytes, reduces surface tension in the alveoli. What is the primary physiological consequence of reduced alveolar surface tension?

• (A) Increased lung compliance
• (B) Decreased lung compliance
• (C) Increased airway resistance
• (D) Decreased functional residual capacity (FRC)

What is the direct effect of antidiuretic hormone (ADH) on the principal cells of the collecting duct?

• (A) Insertion of aquaporin-2 channels into the apical membrane
• (B) Increased activity of the Na+-K+-ATPase pump
• (C) Increased synthesis of epithelial sodium channels (ENaC)
• (D) Inhibition of urea transporters

Which segment of the nephron is the primary site for regulation of calcium excretion by parathyroid hormone (PTH)?

• (A) Distal convoluted tubule (DCT)
• (B) Proximal convoluted tubule (PCT)
• (C) Thick ascending limb of the Loop of Henle
• (D) Collecting duct

During the isovolumetric contraction phase of the cardiac cycle, which of the following is true?

• (A) All four heart valves are closed
• (B) The mitral valve is open, and the aortic valve is closed
• (C) The aortic valve is open, and the mitral valve is closed
• (D) Both AV valves are open

Which physiological process describes the movement of fluid and solutes from the peritubular capillaries into the renal tubular lumen?

• (A) Tubular secretion
• (B) Tubular reabsorption
• (C) Glomerular filtration
• (D) Countercurrent multiplication

The resting membrane potential of a typical neuron is primarily established by the differential permeability of the membrane to which two ions?

• (A) Potassium (K+) and Sodium (Na+)
• (B) Calcium (Ca2+) and Chloride (Cl-)
• (C) Sodium (Na+) and Calcium (Ca2+)
• (D) Potassium (K+) and Chloride (Cl-)

In the gastrointestinal tract, slow waves are spontaneous oscillations in membrane potential generated by which specialized cells?

• (A) Interstitial cells of Cajal (ICC)
• (B) Enteric neurons
• (C) Smooth muscle cells
• (D) Goblet cells

What is the primary stimulus for the release of secretin from duodenal S cells?

• (A) Acidic chyme entering the duodenum
• (B) Fatty acids in the duodenum
• (C) Amino acids in the duodenum
• (D) Distension of the stomach

The plateau phase (phase 2) of the cardiac ventricular action potential is primarily maintained by the influx of which ion?

• (A) Calcium (Ca2+)
• (B) Sodium (Na+)
• (C) Potassium (K+)
• (D) Chloride (Cl-)

Which lung volume represents the amount of air remaining in the lungs after a maximal expiration?

• (A) Residual Volume (RV)
• (B) Expiratory Reserve Volume (ERV)
• (C) Tidal Volume (TV)
• (D) Inspiratory Reserve Volume (IRV)

Activation of the sympathetic nervous system typically causes which effect on the airways?

• (A) Bronchodilation
• (B) Bronchoconstriction
• (C) Increased mucus secretion
• (D) Decreased respiratory rate

How does the kidney respond to chronic respiratory acidosis?

• (A) Increased reabsorption of bicarbonate (HCO3-)
• (B) Decreased reabsorption of bicarbonate (HCO3-)
• (C) Increased secretion of bicarbonate (HCO3-)
• (D) Decreased secretion of hydrogen ions (H+)

Which factor increases the affinity of hemoglobin for oxygen, shifting the oxygen-hemoglobin dissociation curve to the left?

• (A) Decreased temperature
• (B) Increased 2,3-BPG
• (C) Increased PCO2
• (D) Decreased pH

The digestion of carbohydrates begins in the mouth with the action of which enzyme?

• (A) Salivary amylase
• (B) Pancreatic amylase
• (C) Pepsin
• (D) Lingual lipase

What is the main driving force for water reabsorption across the proximal convoluted tubule epithelium?

• (A) Osmotic gradient created by solute reabsorption (especially Na+)
• (B) Hydrostatic pressure gradient
• (C) Active transport of water
• (D) Action of antidiuretic hormone (ADH)

Which ion channel is primarily responsible for the repolarization phase (phase 3) of the ventricular action potential?

• (A) Voltage-gated potassium channels (delayed rectifier)
• (B) Voltage-gated sodium channels
• (C) Voltage-gated calcium channels (L-type)
• (D) Chloride channels

Myasthenia gravis is an autoimmune disease characterized by antibodies against which structure at the neuromuscular junction?

• (A) Nicotinic acetylcholine receptors (nAChR)
• (B) Voltage-gated calcium channels (presynaptic)
• (C) Acetylcholinesterase (AChE)
• (D) Muscarinic acetylcholine receptors (mAChR)

What is the physiological term for the volume of air inhaled or exhaled during a normal quiet breath?

• (A) Tidal Volume (TV)
• (B) Inspiratory Capacity (IC)
• (C) Functional Residual Capacity (FRC)
• (D) Vital Capacity (VC)

The reabsorption of bicarbonate (HCO3-) in the proximal convoluted tubule is primarily dependent on the activity of which enzyme in the tubular cells and lumen?

• (A) Carbonic anhydrase
• (B) Na+-K+-ATPase
• (C) H+-K+-ATPase
• (D) Lactase

What effect does insulin have on potassium levels in the extracellular fluid?

• (A) Causes potassium to shift into cells
• (B) Causes potassium to shift out of cells
• (C) Increases renal excretion of potassium
• (D) Decreases renal excretion of potassium

Which part of the autonomic nervous system controls pupillary constriction (miosis)?

• (A) Parasympathetic nervous system
• (B) Sympathetic nervous system
• (C) Somatic nervous system
• (D) Enteric nervous system

Venous return to the heart is increased by which of the following factors?

• (A) Increased skeletal muscle pump activity
• (B) Increased intrathoracic pressure
• (C) Decreased blood volume
• (D) Increased peripheral venous compliance

Inulin clearance is often used to estimate Glomerular Filtration Rate (GFR) because inulin is:

• (A) Freely filtered, not reabsorbed, and not secreted
• (B) Freely filtered and partially reabsorbed
• (C) Freely filtered and partially secreted
• (D) Not filtered but actively secreted

Which cells in the stomach are responsible for secreting hydrochloric acid (HCl) and intrinsic factor?

• (A) Parietal cells
• (B) Chief cells
• (C) G cells
• (D) Mucous neck cells

What is the primary function of the Loop of Henle in the nephron?

• (A) Generation of the medullary osmotic gradient (countercurrent multiplication)
• (B) Major site of glucose reabsorption
• (C) Secretion of potassium
• (D) Fine-tuning of water reabsorption via ADH

During the rapid ejection phase of the cardiac cycle, which valves are open and which are closed?

• (A) Aortic and pulmonary valves open; Mitral and tricuspid valves closed
• (B) Mitral and tricuspid valves open; Aortic and pulmonary valves closed
• (C) All four valves are closed
• (D) Aortic valve open; Mitral, tricuspid, and pulmonary valves closed

Which hormone acts on the late distal tubule and collecting duct to increase sodium reabsorption and potassium secretion?

• (A) Aldosterone
• (B) Antidiuretic hormone (ADH)
• (C) Angiotensin II
• (D) Atrial natriuretic peptide (ANP)

What is the term for the maximum rate at which a substance can be reabsorbed from the renal tubules?

• (A) Transport maximum (Tm)
• (B) Renal plasma flow (RPF)
• (C) Glomerular filtration rate (GFR)
• (D) Filtration fraction (FF)

The Frank-Starling mechanism of the heart describes the relationship between:

• (A) Stroke volume and end-diastolic volume
• (B) Heart rate and contractility
• (C) Cardiac output and peripheral resistance
• (D) Ejection fraction and afterload

Which component of the extracellular fluid compartment has the highest protein concentration?

• (A) Plasma
• (B) Interstitial fluid
• (C) Lymph
• (D) Transcellular fluid

What physiological change occurs at the onset of puberty, leading to the development of secondary sexual characteristics?

• (A) Increased pulsatile secretion of Gonadotropin-releasing hormone (GnRH)
• (B) Decreased sensitivity of the hypothalamus to estrogen/testosterone negative feedback
• (C) Increased secretion of prolactin
• (D) Decreased secretion of adrenal androgens

Which type of cell junction allows for direct electrical and metabolic coupling between adjacent cells, commonly found in cardiac muscle?

• (A) Gap junctions
• (B) Tight junctions
• (C) Desmosomes
• (D) Adherens junctions