Which of the following is the primary mechanism that drives sodium reabsorption in the proximal tubule?
Which of the following is the most accurate measure of Glomerular Filtration Rate (GFR)?
Normal renal threshold for glucose is at plasma glucose level ?
What is the primary solute responsible for the hyperosmolarity of the renal medulla?
What is the most important extracellular buffer?
Which hormone is secreted by the "Delta cells" of the stomach?
Sugars are primarily absorbed in?
Lowest pH is seen in which of the gastrointestinal secretions?
Daily salivary secretion is
Inhibition of myenteric plexus results in
NEET-PG 2013 - Physiology NEET-PG Practice Questions and MCQs
Question 61: Which of the following is the primary mechanism that drives sodium reabsorption in the proximal tubule?
- A. Sodium reabsorption through cotransport with amino acids at the luminal membrane.
- B. Sodium reabsorption through cotransport with glucose at the luminal membrane.
- C. Sodium reabsorption through countertransport with hydrogen ions at the luminal membrane.
- D. Active sodium transport via the Na+-K+-ATPase pump at the basolateral membrane. (Correct Answer)
Explanation: ***Active sodium transport via the Na+-K+-ATPase pump at the basolateral membrane.*** - This pump **actively transports sodium out of the cell** into the interstitial fluid, creating a low intracellular sodium concentration. - The **Na+-K+-ATPase** is the primary driver of sodium reabsorption throughout the nephron, creating the electrochemical gradient for other sodium transporters. *Sodium reabsorption through cotransport with amino acids at the luminal membrane.* - While **sodium-amino acid cotransport** does occur in the proximal tubule, it accounts for only a fraction of total sodium reabsorption. - The primary driving force for this cotransport is the **low intracellular sodium concentration** maintained by the Na+-K+-ATPase. *Sodium reabsorption through cotransport with glucose at the luminal membrane.* - **Sodium-glucose cotransporters (SGLTs)** are crucial for glucose reabsorption in the proximal tubule, moving glucose into the cell along with sodium. - However, glucose cotransport represents a specific mechanism for glucose handling, not the overarching mechanism for sodium reabsorption. *Sodium reabsorption through countertransport with hydrogen ions at the luminal membrane.* - The **Na+-H+ exchanger (NHE3)** is significant for exchanging sodium for hydrogen ions at the luminal membrane in the proximal tubule. - This mechanism is important for **acid-base balance** and some sodium reabsorption, but it is secondary to the Na+-K+-ATPase in driving the overall sodium gradient.
Question 62: Which of the following is the most accurate measure of Glomerular Filtration Rate (GFR)?
- A. Cystatin C
- B. Serum creatinine
- C. Creatinine Clearance
- D. Iothalamate Clearance (Correct Answer)
Explanation: ***Iothalamate Clearance*** - **Iothalamate clearance** is considered the **gold standard** for directly measuring GFR in clinical practice because it is a substance that is freely filtered by the glomerulus and is neither reabsorbed nor secreted by the renal tubules. - This method provides the most accurate and precise assessment of kidney function by quantifying the actual GFR, often used in research settings or for precise diagnosis. - **Note:** Inulin clearance is the traditional reference standard, but iothalamate is more practical and widely used clinically as it can be measured using radioactive or non-radioactive methods. *Serum creatinine* - **Serum creatinine** is a commonly used biomarker but is an **imperfect measure** of GFR because it can be influenced by factors like muscle mass, diet, and certain medications. - Its levels can remain within the normal range even when GFR has significantly decreased, especially in the early stages of kidney disease. *Cystatin C* - **Cystatin C** is a protein produced by most nucleated cells and is also freely filtered by the glomerulus, with less influence from muscle mass and diet compared to creatinine. - While considered a better marker than serum creatinine, it is still an **estimated measure** and is more expensive and less widely available than creatinine, and can be affected by inflammation or thyroid dysfunction. *Creatinine Clearance* - **Creatinine clearance** (often estimated using urine and serum creatinine levels over a timed collection) attempts to approximate GFR but can be **inaccurate** due to incomplete urine collection and tubular secretion of creatinine. - The **creatinine secretion** by the renal tubules leads to an overestimation of the true GFR, making it less accurate than direct measurement methods.
Question 63: Normal renal threshold for glucose is at plasma glucose level ?
- A. 100 mg/dl
- B. 200 mg/dl (Correct Answer)
- C. 300 mg/dl
- D. 400 mg/dl
Explanation: ** _200 mg/dl_ ** - The **renal threshold for glucose** represents the plasma glucose concentration at which the kidneys begin to excrete glucose into the urine. - This typically occurs when the glucose level exceeds the reabsorptive capacity of the renal tubules, usually around **180-200 mg/dL**. * _100 mg/dl_ * - A plasma glucose level of **100 mg/dL** is within the normal fasting range and well below the renal threshold. - At this level, virtually all filtered glucose is reabsorbed by the renal tubules, and no glucose appears in the urine. * _300 mg/dl_ * - A plasma glucose level of **300 mg/dL** is significantly above the renal threshold for glucose. - At this concentration, the kidney's reabsorptive capacity is overwhelmed, leading to substantial **glucosuria** (glucose in the urine). * _400 mg/dl_ * - A plasma glucose level of **400 mg/dL** is severely elevated and far exceeds the renal threshold. - This level would result in significant glucose excretion in the urine and is indicative of uncontrolled hyperglycemia, as seen in **diabetes mellitus**.
Question 64: What is the primary solute responsible for the hyperosmolarity of the renal medulla?
- A. urea
- B. K
- C. Na (Correct Answer)
- D. Cl
Explanation: ***Na*** - **Sodium (Na+), along with chloride**, is the primary solute responsible for establishing the **corticomedullary osmotic gradient** in the renal medulla. - Actively reabsorbed in the **thick ascending limb of the loop of Henle** via the Na-K-2Cl cotransporter, creating hyperosmolarity in the outer medulla. - NaCl accounts for the majority of osmolality in the **outer medulla** and provides the foundation for the countercurrent multiplication system. - While **urea contributes significantly to inner medullary hyperosmolarity** (especially during antidiuresis), **sodium chloride** is considered the **primary driving force** for the overall medullary concentration gradient. *K* - **Potassium (K+)** is primarily involved in maintaining intracellular fluid balance and cellular membrane potentials. - While K+ is reabsorbed in the loop of Henle (via Na-K-2Cl cotransporter), it does not accumulate in the medullary interstitium to contribute significantly to hyperosmolarity. *urea* - **Urea** contributes substantially to hyperosmolarity, particularly in the **inner medulla** (accounting for ~40-50% of inner medullary osmolality). - Through **urea recycling** (collecting duct → medullary interstitium → thin limbs), it enhances urinary concentration, especially during water deprivation. - However, the **initial establishment** of the osmotic gradient depends on **NaCl reabsorption** in the ascending limb, making sodium the primary solute. *Cl* - **Chloride (Cl-)** is reabsorbed together with sodium via the Na-K-2Cl cotransporter in the thick ascending limb. - Functionally, **NaCl works as a unit** to create medullary hyperosmolarity, so chloride and sodium are inseparable in this process. - Among the listed options, **sodium** represents this NaCl contribution as the cation driving active transport.
Question 65: What is the most important extracellular buffer?
- A. Bicarbonates (Correct Answer)
- B. Phosphate buffer
- C. Plasma protein buffer
- D. Ammonium buffer
Explanation: ***Bicarbonates*** - The **bicarbonate buffer system** is the most important extracellular buffer because its components (carbonic acid and bicarbonate) are present in high concentrations and their levels can be regulated by both the lungs (CO2 excretion) and the kidneys (bicarbonate reabsorption/secretion). - Its pKa (6.1) makes it an effective buffer against metabolically produced acids, which frequently challenge blood pH. *Phosphate buffer* - The **phosphate buffer system** is more important as an intracellular buffer and in renal tubular fluid due to its higher concentration in these compartments. - Its concentration in the extracellular fluid is relatively low compared to bicarbonate, limiting its capacity as the primary extracellular buffer. *Plasma protein buffer* - **Plasma proteins**, particularly albumin, have numerous ionizable groups and contribute to buffering in the extracellular fluid. - However, their overall buffering capacity is less significant than that of the bicarbonate system due to lower concentration compared to bicarbonate and less dynamic regulation. *Ammonium buffer* - The **ammonium buffer system** (ammonia/ammonium) is primarily important for acid-base regulation by the kidneys, where it plays a critical role in excreting excess acid, particularly in chronic acidosis. - It is not a major extracellular fluid buffer in the systemic circulation under normal physiological conditions.
Question 66: Which hormone is secreted by the "Delta cells" of the stomach?
- A. Cholecystokinin
- B. Gastrin-releasing peptide
- C. Somatostatin (Correct Answer)
- D. Secretin
Explanation: ***Somatostatin*** - **Delta cells (D cells)** in the stomach and pancreas secrete **somatostatin**, a potent inhibitory hormone. - Somatostatin **inhibits the release of gastrin**, histamine, secretin, cholecystokinin, and gastric acid secretion, acting as a "universal off switch." *Cholecystokinin* - **Cholecystokinin (CCK)** is primarily secreted by **I cells** in the duodenum and jejunum. - Its main functions include stimulating gallbladder contraction and pancreatic enzyme secretion. *Gastrin-releasing peptide* - **Gastrin-releasing peptide (GRP)**, also known as **bombesin**, is a neuropeptide released from **enteric neurons**. - It stimulates the release of **gastrin** from G cells. *Secretin* - **Secretin** is secreted by **S cells** in the duodenum in response to acidic chyme entering the small intestine. - Its primary role is to stimulate the pancreas to release **bicarbonate-rich fluid** to neutralize gastric acid.
Question 67: Sugars are primarily absorbed in?
- A. Duodenum
- B. Jejunum (Correct Answer)
- C. Ascending colon
- D. Ileum
Explanation: ***Jejunum*** - The **jejunum** is the primary site for the absorption of most digested nutrients, including the vast majority of **monosaccharides** (simple sugars like glucose, fructose, and galactose). - Its structure, with numerous **plicae circulares**, villi, and microvilli, provides a large surface area optimized for efficient nutrient uptake. *Duodenum* - The **duodenum** is mainly involved in the **chemical digestion** of food, receiving chyme from the stomach and mixing it with digestive enzymes from the pancreas and bile from the liver. - While some minimal absorption can occur, it is not the primary site for extensive sugar absorption. *Ileum* - The **ileum** is mainly responsible for the absorption of **vitamin B12** and **bile salts**. - Although some residual nutrient absorption can happen here if the jejunum is compromised, it is not the primary physiological site for sugar absorption. *Ascending colon* - The **ascending colon** is primarily involved in the absorption of **water and electrolytes**, forming solid stool. - It does not significantly absorb sugars; undigested carbohydrates reaching the colon are typically fermented by gut bacteria.
Question 68: Lowest pH is seen in which of the gastrointestinal secretions?
- A. Gastric juice (Correct Answer)
- B. Bile juice
- C. Saliva
- D. Pancreatic juice
Explanation: ***Gastric juice*** - Gastric juice contains **hydrochloric acid (HCl)**, secreted by parietal cells, which gives it a very **low pH (1.5-3.5)**. - This acidic environment is crucial for protein digestion by **pepsin** and for killing ingested microorganisms. *Bile juice* - Bile juice is typically **alkaline**, with a pH ranging from **7.6 to 8.6**. - Its primary role is to **emulsify fats** in the small intestine, and it does not contain significant acidic components. *Saliva* - Saliva has a relatively neutral pH, typically ranging from **6.2 to 7.6**. - It contains enzymes like **amylase** and **lipase** for initial carbohydrate and lipid digestion, but no strong acids. *Pancreatic juice* - Pancreatic juice is highly **alkaline**, with a pH usually between **8.0 and 8.3**, due to its high concentration of bicarbonate. - This alkalinity neutralizes the acidic chyme entering the duodenum from the stomach, creating an optimal environment for pancreatic enzymes.
Question 69: Daily salivary secretion is
- A. 1000-1500 ml (Correct Answer)
- B. 1500-2000 ml
- C. More than 2000 ml
- D. Less than 1000 ml
Explanation: ***1000-1500 ml*** - The average daily salivary secretion in healthy adults ranges from **1000 to 1500 ml**, with variations depending on individual factors and stimulation. - This volume is crucial for various functions, including **digestion**, oral hygiene, and speech. *1500-2000 ml* - This range is generally considered to be on the **higher side** of normal daily salivary output, exceeding the typical average. - While individual variations exist, consistent secretion at this level might suggest **hypersecretion** or ptyalism in some cases. *More than 2000 ml* - Daily salivary secretion **rarely exceeds 2000 ml** in healthy individuals. - Such high volumes could indicate a pathological condition leading to **sialorrhea** or excessive salivation. *Less than 1000 ml* - A daily salivary secretion of **less than 1000 ml** is often indicative of **hyposalivation** or dry mouth (xerostomia). - This reduced volume can lead to problems with chewing, swallowing, speaking, and an increased risk of dental caries.
Question 70: Inhibition of myenteric plexus results in
- A. Hyperacidity
- B. Diarrhea
- C. Decreased gut motility (Correct Answer)
- D. Increased secretions
Explanation: ***Decreased gut motility*** - The **myenteric plexus** (Auerbach's plexus) is primarily responsible for regulating **gastrointestinal motility**, including peristalsis and muscle contraction. - Its inhibition would therefore lead to **reduced peristaltic movements** and **decreased gut motility**. *Hyperacidity* - **Gastric acid secretion** is mainly regulated by the vagus nerve (via acetylcholine), gastrin, and histamine, not directly by the myenteric plexus. - While gut motility can indirectly affect acid exposure, a primary and direct consequence of myenteric plexus inhibition is not hyperacidity. *Diarrhea* - **Diarrhea** is typically caused by increased gut motility, increased secretion, or decreased absorption. - Inhibition of the myenteric plexus would lead to **decreased motility**, making diarrhea an unlikely outcome. *Increased secretions* - **Gastrointestinal secretions** are largely controlled by the submucosal plexus (Meissner's plexus) and hormonal factors. - While the myenteric plexus has some indirect influence, its primary role is motility, and its inhibition would not directly lead to increased secretions.