USMLE PRACTICE QUESTION #100
TOPIC: Renal Physiology.
Question #100 will test your understanding of the basics of renal physiology.
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A 55-year-old male with a history of diabetes mellitus presents to the clinic complaining of fatigue and decreased urine output. His blood pressure is 100/65 mmHg and heart rate is 98/minute. Which of the following mechanisms will help him maintain adequate blood flow to the kidneys?
A) Prostaglandins will vasoconstrict the renal vessels
B) Macula dense will dilate the afferent arterioles in response to hypertonic urine
C) The RAAS will stimulate the thirst mechanism in the pituitary
D) Myogenic autoregulation will dilate the afferent arterioles
E) JG apparatus will respond to decreased pressure within DCT
Detailed Breakdown of Answers + Correct Answer Below ⏬
ANSWER + QUESTION BREAKDOWN
The MENTAL MODEL used to answer this question comes from our detailed test-taking skills masterclass (check it out if you want to elevate your skills). Here’s how to think through this question:
Step 1. Read the last line to get to the heart of the question: “Which of the following mechanisms will help him maintain adequate blood flow to the kidneys?”
Step 2: Is this a first-, second-, or third-order question?
Answer: 2nd order. 1st: Identify the patient’s current status; 2nd: Based on status, recognize how renal blood flow is maintained.
Step 3: Read the vignette carefully and ask yourself: “Based on the patient’s state, how is renal blood flow maintained?”
Step 4. Look at the answer choices and select the option most closely resembling your final thought from “Step 3” above.
GENERAL ANALYSIS
This vignette describes a 55-year-old male with a history of diabetes mellitus who presents with fatigue and decreased urine output, suggesting possible renal hypoperfusion or early kidney dysfunction. His blood pressure is relatively low (100/65 mmHg), and his heart rate is mildly elevated (98/min), both of which suggest that his body may be compensating for decreased perfusion. The question asks which mechanism will help maintain adequate blood flow to the kidneys, focusing on renal autoregulation and related physiological responses.
ANSWER CHOICES:
CHOICE A: Prostaglandins will vasoconstrict the renal vessels
Explanation: Prostaglandins, particularly prostaglandin E2 (PGE2), play a protective role in the kidneys by causing vasodilation of the afferent arterioles, helping to maintain renal blood flow, especially during states of hypoperfusion. Therefore, this answer is incorrect.
CHOICE B: Macula densa will dilate the afferent arterioles in response to hypertonic urine
Explanation: The macula densa cells are part of the juxtaglomerular apparatus and respond to changes in sodium concentration in the distal convoluted tubule. When there is reduced sodium delivery (often due to decreased glomerular filtration rate), they signal for afferent arteriole dilation to increase renal blood flow. This answer is incorrect since the macula densa responds to hypotonic urine rather than hypertonic urine.
CHOICE C: The RAAS will stimulate the thirst mechanism in the pituitary
Explanation: The renin-angiotensin-aldosterone system (RAAS) is activated in response to low blood pressure or decreased renal perfusion. While angiotensin II can stimulate thirst via effects on the hypothalamus, it does not directly stimulate thirst via the pituitary gland. The thirst mechanism is primarily controlled by the hypothalamus, not the pituitary gland. Additionally, this does not directly address how renal blood flow is maintained. Therefore, this answer is incorrect.
CHOICE D: Myogenic autoregulation will dilate the afferent arterioles
Explanation: The myogenic mechanism is one of the key autoregulatory processes that helps maintain consistent renal blood flow despite changes in systemic blood pressure. When blood pressure drops, smooth muscle cells in the afferent arterioles relax, leading to vasodilation and increased renal blood flow.
CHOICE E: JG apparatus will respond to decreased pressure within DCT
Explanation: The juxtaglomerular (JG) apparatus responds to changes in blood pressure and sodium concentration. However, it responds primarily to decreased perfusion pressure in the afferent arteriole or low sodium delivery to the distal convoluted tubule (DCT), not directly to pressure within the DCT itself. The JG apparatus does not respond to pressure within the DCT but rather to sodium concentration and perfusion pressure changes upstream. Therefore, this answer is incorrect.
FINAL VERDICT…
The kidneys have intrinsic mechanisms that regulate renal blood flow and glomerular filtration rate (GFR) over a wide range of systemic blood pressures. One such mechanism is myogenic autoregulation, which involves smooth muscle cells in the walls of afferent arterioles responding to changes in stretch caused by fluctuations in blood pressure.
When systemic blood pressure drops, as seen in this patient with relatively low BP (100/65 mmHg), these smooth muscle cells relax, dilating the afferent arterioles. This helps maintain adequate renal perfusion and GFR despite reduced systemic pressure.
This process is crucial for maintaining kidney function during periods of hypoperfusion or hypotension, as it ensures that metabolic wastes are still filtered from the bloodstream into urine while preventing excessive fluid loss when GFR might otherwise drop too low.
Thus, myogenic autoregulation plays a crucial role in maintaining kidney function under conditions like those described in this patient.
CORRECT ANSWER: D) Myogenic autoregulation will dilate the afferent arterioles
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That’s it for question 100!
See ya tomorrow 👋