A researcher is studying the effect of hypothalamic lesions on rat behavior and development. She has developed a novel genetic engineering technology that allows her to induce specific mutations in rat embryos that interfere with rat CNS development. She creates several lines of rats with mutations in only one region of the pituitary gland and hypothalamus. She then monitors their growth and development over six weeks. One line of rats has a mean body mass index (BMI) that is significantly lower than that of control rats. Food intake decreased by 40% in these rats compared to controls. These rats likely have a lesion in which of the following locations?
Q22
In an experimental model, a compound is centrally administered to mice. Following administration, the mice display increased desire for food and increased appetite. The administered compound is most likely similar to which of the following?
Q23
A 62-year-old man is found unconscious in the park on a bench, early in the morning in January. The temperature outside is -4.0°C (25°F). He is barefoot and is wearing nothing more than sweatpants, a tee-shirt, and a light coat. Upon arrival at the emergency department, his vitals include: heart rate 45/min, blood pressure 100/70 mm Hg, and respiratory rate 10/min. His core body temperature is 30.0°C (85.5°F). His feet and palms are covered with clear blisters, the skin is yellow with a waxy appearance, and the tissues are edematous. The patient is unresponsive to auditory stimuli. Which of the following cold-associated injuries does the patient have?
Q24
In large neurons the nucleus can be found a large distance away from the terminal end of its axon. The body has a complex system of intracellular transporters that are able to carry essential proteins from the nucleus to the distal edge of the cell and back. Which of the following proteins are essential for this function?
Q25
A 17-year-old previously healthy, athletic male suddenly falls unconscious while playing soccer. His athletic trainer comes to his aid and notes that he is pulseless. He begins performing CPR on the patient until the ambulance arrives but the teenager is pronounced dead when the paramedics arrived. Upon investigation of his primary care physician's office notes, it was found that the child had a recognized murmur that was ruled to be "benign." Which of the following conditions would have increased the intensity of the murmur?
Q26
One day after undergoing surgery for a traumatic right pelvic fracture, a 73-year-old man has pain over his buttocks and scrotum and urinary incontinence. Physical examination shows right-sided perineal hypesthesia and absence of anal sphincter contraction when the skin around the anus is touched. This patient is most likely to have which of the following additional neurological deficits?
Q27
A 56-year-old woman presents to the emergency department after falling in the shower 2 days ago. At that time, she was complaining of severe back pain that she treated with over the counter acetaminophen. Since the fall, she has had increasing stomach pain and difficulty walking. She has not urinated or had a bowel movement since the incident. She has no significant past medical history and takes a multivitamin regularly. No significant family history. Her vitals include: blood pressure 137/82 mm Hg, pulse 78/min, respiratory rate 16/min, temperature 37.0°C (98.6°F). On physical examination, she is alert and oriented but in great discomfort. There is focal, non-radiating midline pain with palpation of the L1 vertebrae. On pinprick sensory testing, she reveals decreased sensations below the level of the knees bilaterally. Muscle strength is 4/5 in both lower extremities. Which of the following additional findings would most likely be present in this patient?
Q28
A 48-year-old woman is brought to the emergency department by her family at her psychiatrist's recommendation. According to her family, she has been more restless than her baseline over the past week. The patient herself complains that she feels her mind is racing. Her past medical history is significant for bipolar disorder on lithium and type 1 diabetes mellitus. The family and the patient both assert that the patient has been taking her medications. She denies any recent illness or sick contacts. The patient's temperature is 100°F (37.8°C), blood pressure is 100/60 mmHg, pulse is 130/min, and respirations are 20/min. She appears diaphoretic, and her cardiac exam is notable for an irregularly irregular rhythm with a 2/6 early systolic murmur. Blood counts and metabolic panel are within normal limits. The patient's lithium level is within therapeutic range. Which of the following laboratory tests would be the most useful to include in the evaluation of this patient?
Q29
An investigator is studying the crossbridge cycle of muscle contraction. Tissue from the biceps brachii muscle is obtained at the autopsy of an 87-year-old man. Investigation of the muscle tissue shows myosin heads attached to actin filaments. Binding of myosin heads to which of the following elements would most likely cause detachment of myosin from actin filaments?
Q30
An investigator is working with a mutant strain of mice that lack a consistent density of sarcolemmal transverse tubules in the skeletal muscle cells. Which of the following is the most likely associated finding as a result of this abnormality?
Cardiovascular US Medical PG Practice Questions and MCQs
Question 21: A researcher is studying the effect of hypothalamic lesions on rat behavior and development. She has developed a novel genetic engineering technology that allows her to induce specific mutations in rat embryos that interfere with rat CNS development. She creates several lines of rats with mutations in only one region of the pituitary gland and hypothalamus. She then monitors their growth and development over six weeks. One line of rats has a mean body mass index (BMI) that is significantly lower than that of control rats. Food intake decreased by 40% in these rats compared to controls. These rats likely have a lesion in which of the following locations?
A. Lateral hypothalamus (Correct Answer)
B. Suprachiasmatic nucleus
C. Ventromedial area
D. Paraventricular nucleus
E. Posterior hypothalamus
Explanation: ***Lateral hypothalamus***
- The **lateral hypothalamus** is known as the **"hunger center"**. Lesions in this area lead to **aphagia** (decreased food intake) and **weight loss**.
- Decreased food intake by 40% and significantly lower BMI are consistent with a lesion affecting the lateral hypothalamus.
*Suprachiasmatic nucleus*
- The **suprachiasmatic nucleus** is the primary regulator of **circadian rhythms** and sleep-wake cycles.
- Lesions here would primarily affect sleep patterns and daily physiological rhythms, not directly cause severe reductions in food intake or BMI.
*Ventromedial area*
- The **ventromedial hypothalamus** is considered the **"satiety center"**. Lesions in this region typically lead to **hyperphagia** (increased food intake) and **obesity**.
- This effect is the opposite of what is observed in the rat line described in the question.
*Paraventricular nucleus*
- The **paraventricular nucleus** plays a role in various functions, including stress response, metabolism, and **feeding behavior**, but its lesion mostly causes increased feeding specifically of carbohydrates.
- While it's involved in feeding, severe aphagia and dramatic weight loss are more classically associated with lateral hypothalamic lesions.
*Posterior hypothalamus*
- The **posterior hypothalamus** is involved in temperature regulation and arousal.
- Lesions in this area can cause problems with **thermoregulation** and can lead to somnolence, but are not directly associated with a primary reduction in food intake and BMI.
Question 22: In an experimental model, a compound is centrally administered to mice. Following administration, the mice display increased desire for food and increased appetite. The administered compound is most likely similar to which of the following?
A. Leptin
B. Neuropeptide-Y (Correct Answer)
C. Cholecystokinin
D. Glucagon-like peptide 1
E. Peptide YY
Explanation: ***Neuropeptide-Y***
- **Neuropeptide Y (NPY)** is a potent **orexigenic peptide** that, when centrally administered, stimulates appetite and increases food intake.
- Its actions primarily occur within the **hypothalamus**, where it promotes a state of positive energy balance.
*Leptin*
- **Leptin** is an **anorexigenic hormone** produced by adipose tissue that signals satiety to the brain, thereby *reducing* appetite and food intake.
- Administration of leptin would lead to *decreased* desire for food, directly opposite to the scenario described.
*Cholecystokinin*
- **Cholecystokinin (CCK)** is a **satiety hormone** released in response to nutrient presence in the small intestine, acting to *reduce* food intake.
- It slows gastric emptying and sends signals of fullness to the brain, thus *decreasing* appetite.
*Glucagon-like peptide 1*
- **Glucagon-like peptide 1 (GLP-1)** is an **incretin hormone** that *reduces* appetite and promotes satiety by slowing gastric emptying and increasing insulin secretion.
- Its central administration would lead to *decreased* food desire, not increased.
*Peptide YY*
- **Peptide YY (PYY)** is an **anorexigenic hormone** released post-prandially from the gastrointestinal tract, signaling satiety and *reducing* appetite.
- Central administration of PYY would result in a *decrease* in food intake and desire.
Question 23: A 62-year-old man is found unconscious in the park on a bench, early in the morning in January. The temperature outside is -4.0°C (25°F). He is barefoot and is wearing nothing more than sweatpants, a tee-shirt, and a light coat. Upon arrival at the emergency department, his vitals include: heart rate 45/min, blood pressure 100/70 mm Hg, and respiratory rate 10/min. His core body temperature is 30.0°C (85.5°F). His feet and palms are covered with clear blisters, the skin is yellow with a waxy appearance, and the tissues are edematous. The patient is unresponsive to auditory stimuli. Which of the following cold-associated injuries does the patient have?
A. Trench foot
B. Pernio
C. Frostbite (Correct Answer)
D. Frostnip
E. Immersion foot
Explanation: ***Frostbite***
- The presence of **clear blisters**, **yellow, waxy skin**, and **edema** in a severely cold and underdressed patient at -4.0°C (25°F) is highly indicative of frostbite. Frostbite is a tissue injury caused by **freezing of tissue** at or below 0°C (32°F).
- The patient's core body temperature of 30.0°C (85.5°F) indicates **hypothermia**, which often co-occurs with frostbite due to prolonged cold exposure.
*Trench foot*
- **Trench foot** (or non-freezing cold injury) occurs from prolonged exposure to **wet and cold** (but not freezing) conditions, typically between 0°C and 15°C (32°F-60°F).
- While it can cause blistering and tissue damage, the characteristic **waxy appearance** and exposure to sub-freezing temperatures make frostbite a more accurate diagnosis.
*Pernio*
- **Pernio**, also known as **chilblains**, are small, itchy, painful, red, or purple skin swellings that occur on the skin surface due to **repeated exposure to cold but non-freezing temperatures**.
- This condition is typically less severe than frostbite and does not involve the deep tissue changes or extensive blistering described.
*Frostnip*
- **Frostnip** is a mild, superficial cold injury that involves the **freezing of the outer layers of skin** but does not cause permanent tissue damage.
- Symptoms include numbness, tingling, and pale skin, but it typically does not involve **blister formation** or the waxy appearance seen in more severe cold injuries like frostbite.
*Immersion foot*
- **Immersion foot** is essentially the same condition as **trench foot**, resulting from prolonged exposure to wet and cold conditions (above freezing), leading to nerve and tissue damage.
- The patient's exposure to **sub-freezing temperatures** and the specific skin changes point away from immersion foot and toward freezing-related injury.
Question 24: In large neurons the nucleus can be found a large distance away from the terminal end of its axon. The body has a complex system of intracellular transporters that are able to carry essential proteins from the nucleus to the distal edge of the cell and back. Which of the following proteins are essential for this function?
A. Kinesin, Troponin
B. Myosin, Kinesin
C. Actin, Dynein
D. Dynein, Kinesin (Correct Answer)
E. Glucose, Actin
Explanation: ***Dynein, Kinesin***
- **Kinesin** is primarily responsible for **anterograde transport** (from the cell body to the axon terminal) along microtubules, carrying vesicles and organelles.
- **Dynein** handles **retrograde transport** (from the axon terminal back to the cell body), essential for recycling components and signaling.
*Kinesin, Troponin*
- While **Kinesin** is involved in axonal transport, **Troponin** is a protein found in muscle tissue that regulates muscle contraction, not intracellular transport in neurons.
- Troponin binds **calcium ions** and influences the interaction between actin and myosin.
*Myosin, Kinesin*
- **Kinesin** is involved in microtubule-based transport, but **Myosin** is primarily associated with **actin filaments** for muscle contraction and intracellular movement, not long-distance axonal transport.
- Myosin functions as a **motor protein** that converts chemical energy in ATP into mechanical force.
*Actin, Dynein*
- **Dynein** is crucial for retrograde axonal transport, but **Actin** is a structural protein forming microfilaments that are involved in cell shape, motility, and some short-distance transport, not the major long-distance axonal transport mechanism.
- Actin filaments serve as tracks for **myosin motors**, primarily in the cell cortex.
*Glucose, Actin*
- **Glucose** is a sugar molecule, the primary energy source for cells, and not a transport protein.
- **Actin** forms microfilaments for cell structure and short-range movement, not long-distance axonal transport as described.
Question 25: A 17-year-old previously healthy, athletic male suddenly falls unconscious while playing soccer. His athletic trainer comes to his aid and notes that he is pulseless. He begins performing CPR on the patient until the ambulance arrives but the teenager is pronounced dead when the paramedics arrived. Upon investigation of his primary care physician's office notes, it was found that the child had a recognized murmur that was ruled to be "benign." Which of the following conditions would have increased the intensity of the murmur?
A. Inspiration
B. Placing the patient in a squatting position
C. Valsalva (Correct Answer)
D. Passive leg raise
E. Handgrip
Explanation: ***Valsalva***
- The patient's sudden death after collapsing during soccer, coupled with a previously noted "benign" murmur, strongly suggests **hypertrophic obstructive cardiomyopathy (HOCM)**, which is a common cause of sudden cardiac death in young athletes. The **Valsalva maneuver** decreases preload and left ventricular volume, thereby **increasing the left ventricular outflow tract (LVOT) obstruction** and hence the intensity of the HOCM murmur.
- This maneuver reduces venous return to the heart, leading to reduced ventricular filling and decreased stroke volume. This exacerbates the obstruction in HOCM, making the murmur louder.
*Inspiration*
- **Inspiration** typically **increases venous return to the right side of the heart**, which would generally intensify right-sided murmurs (e.g., tricuspid regurgitation).
- It would have **minimal effect or slightly decrease** the intensity of a left-sided obstructive murmur like that in HOCM, as it does not directly increase the LVOT obstruction.
*Placing the patient in a squatting position*
- Squatting increases both **preload** and **afterload** by increasing systemic vascular resistance and venous return.
- This increase in ventricular volume would **reduce the outflow tract obstruction** in HOCM, thereby **decreasing the intensity of the murmur**.
*Passive leg raise*
- A **passive leg raise** increases **venous return** and thus **preload**, leading to increased ventricular filling.
- Similar to squatting, this increased left ventricular volume would **reduce the left ventricular outflow tract obstruction** associated with HOCM, thereby **decreasing the murmur's intensity**.
*Handgrip*
- The **handgrip maneuver** primarily **increases afterload** and, to some extent, preload by increasing systemic vascular resistance.
- While it can increase the intensity of murmurs like mitral regurgitation and ventricular septal defect, it would generally **decrease or have no significant effect** on the murmur of HOCM due to the increased ventricular volume reducing the outflow obstruction.
Question 26: One day after undergoing surgery for a traumatic right pelvic fracture, a 73-year-old man has pain over his buttocks and scrotum and urinary incontinence. Physical examination shows right-sided perineal hypesthesia and absence of anal sphincter contraction when the skin around the anus is touched. This patient is most likely to have which of the following additional neurological deficits?
A. Impaired hip flexion
B. Paralysis of hip adductors
C. Absent cremasteric reflex
D. Impaired psychogenic erection
E. Absent reflex erection (Correct Answer)
Explanation: ***Absent reflex erection***
- The patient's symptoms (buttock/scrotal pain, perineal hypesthesia, urinary incontinence, absent anal sphincter contraction) suggest **damage to the sacral plexus and pudendal nerve**, consistent with a **cauda equina syndrome**.
- **Reflex erections** are primarily mediated by the **sacral parasympathetic outflow (S2-S4)**, which are likely compromised given the other sacral nerve deficits.
*Impaired hip flexion*
- **Hip flexion** is primarily controlled by the **L1-L3 nerve roots** (e.g., iliopsoas muscle), and while a severe pelvic fracture could cause widespread nerve damage, the current symptoms localize more strongly to the sacral region.
- The described symptoms are more indicative of **sacral nerve involvement** rather than higher lumbar segments that govern hip flexion.
*Paralysis of hip adductors*
- **Hip adduction** is mainly innervated by the **obturator nerve (L2-L4)**.
- The patient's symptoms point to **S2-S4 nerve dysfunction** (perineal sensation, anal sphincter, bladder), which are distinct from the obturator nerve's primary innervations.
*Absent cremasteric reflex*
- The **cremasteric reflex** is mediated by the **genitofemoral nerve (L1-L2)**.
- The symptoms presented are more consistent with **sacral nerve damage**, specifically S2-S4, rather than the higher lumbar segments responsible for the cremasteric reflex.
*Impaired psychogenic erection*
- **Psychogenic erections** are initiated by **supraspinal input** descending through the thoracolumbar spinal cord (T10-L2) to activate sympathetic pathways.
- While sacral nerve damage can affect the final efferent pathway for all erections, the direct impairment of psychogenic initiation is linked to higher centers and **thoracolumbar sympathetic outflow**, not purely sacral damage.
Question 27: A 56-year-old woman presents to the emergency department after falling in the shower 2 days ago. At that time, she was complaining of severe back pain that she treated with over the counter acetaminophen. Since the fall, she has had increasing stomach pain and difficulty walking. She has not urinated or had a bowel movement since the incident. She has no significant past medical history and takes a multivitamin regularly. No significant family history. Her vitals include: blood pressure 137/82 mm Hg, pulse 78/min, respiratory rate 16/min, temperature 37.0°C (98.6°F). On physical examination, she is alert and oriented but in great discomfort. There is focal, non-radiating midline pain with palpation of the L1 vertebrae. On pinprick sensory testing, she reveals decreased sensations below the level of the knees bilaterally. Muscle strength is 4/5 in both lower extremities. Which of the following additional findings would most likely be present in this patient?
A. Decreased deep tendon reflexes (Correct Answer)
B. Intentional tremors
C. Increased deep tendon reflexes
D. Decreased muscle strength in upper extremities
E. Difficulty in swallowing
Explanation: ***Decreased deep tendon reflexes***
- The patient's presentation with **severe back pain at L1**, **bowel and bladder dysfunction**, **sensory deficits below the knees**, and **difficulty walking** points to a **conus medullaris syndrome** or **cauda equina syndrome**.
- The **spinal cord terminates at approximately L1-L2** (conus medullaris), below which are the nerve roots of the cauda equina.
- Lesions at or below L1 affect the **lower motor neurons (LMN)** directly, producing **hyporeflexia or areflexia** (decreased or absent deep tendon reflexes).
- The **early bowel and bladder dysfunction** combined with **bilateral sensory loss** is characteristic of **conus medullaris syndrome**, which presents with predominantly **LMN signs** including decreased reflexes.
*Increased deep tendon reflexes*
- **Hyperreflexia** is a hallmark of **upper motor neuron (UMN) lesions** affecting the spinal cord above the level of L1-L2.
- At the L1 vertebral level, the spinal cord is terminating (conus medullaris), so compression here affects lower motor neurons, not upper motor neurons.
- UMN signs would only be expected if the lesion were at a higher thoracic or cervical level.
*Intentional tremors*
- **Intention tremors** are associated with **cerebellar dysfunction**, characterized by tremor that worsens with goal-directed movement.
- The patient's symptoms localize to the spinal cord/nerve roots at L1, not the cerebellum.
*Decreased muscle strength in upper extremities*
- The patient's lesion is at **L1 level**, which is far below the **cervical spinal cord segments** (C5-T1) that innervate the upper extremities.
- Upper extremity weakness would only occur with cervical or high thoracic cord lesions.
*Difficulty in swallowing*
- **Dysphagia** results from dysfunction of **cranial nerves IX, X, and XII** or lesions affecting the **brainstem** or **cortical swallowing centers**.
- A spinal lesion at L1 cannot cause swallowing difficulties.
Question 28: A 48-year-old woman is brought to the emergency department by her family at her psychiatrist's recommendation. According to her family, she has been more restless than her baseline over the past week. The patient herself complains that she feels her mind is racing. Her past medical history is significant for bipolar disorder on lithium and type 1 diabetes mellitus. The family and the patient both assert that the patient has been taking her medications. She denies any recent illness or sick contacts. The patient's temperature is 100°F (37.8°C), blood pressure is 100/60 mmHg, pulse is 130/min, and respirations are 20/min. She appears diaphoretic, and her cardiac exam is notable for an irregularly irregular rhythm with a 2/6 early systolic murmur. Blood counts and metabolic panel are within normal limits. The patient's lithium level is within therapeutic range. Which of the following laboratory tests would be the most useful to include in the evaluation of this patient?
A. Thyroglobulin level
B. Thyroid stimulating hormone and total thyroxine levels
C. Thyrotropin-releasing hormone stimulation test
D. Triiodothyronine and thyroxine levels
E. Thyroid stimulating hormone and free thyroxine levels (Correct Answer)
Explanation: ***Thyroid stimulating hormone and free thyroxine levels***
- This patient presents with symptoms highly suggestive of **hyperthyroidism**, including **restlessness**, **tachycardia** (pulse 130/min), **diaphoresis**, and a **racing mind**. These symptoms, particularly in a patient on lithium (which can induce thyroid dysfunction), necessitate a thorough thyroid evaluation.
- While total T4 can be influenced by protein binding, **free T4** is the active form of the hormone and provides a more accurate assessment of thyroid status, along with **TSH** as the primary screening test.
*Thyroglobulin level*
- **Thyroglobulin** is primarily used as a tumor marker in the follow-up of patients treated for differentiated thyroid cancer to detect recurrence.
- It is not a primary diagnostic test for hyperthyroidism and would not be the most useful initial test in this scenario.
*Thyroid stimulating hormone and total thyroxine levels*
- While TSH and total T4 are used, **total T4** levels can be affected by changes in **thyroid-binding globulin (TBG)**, which can be altered by various conditions or medications (e.g., estrogen, liver disease).
- Therefore, **free thyroxine** provides a more accurate reflection of thyroid function than total thyroxine.
*Thyrotropin-releasing hormone stimulation test*
- A **TRH stimulation test** is primarily used to investigate subtle abnormalities in the hypothalamic-pituitary-thyroid axis, often when other thyroid function tests are equivocal.
- It is not a first-line diagnostic test for overt hyperthyroidism and would be overly complex for initial evaluation of this patient's symptoms.
*Triiodothyronine and thyroxine levels*
- While T3 and T4 levels are part of thyroid function assessment, specifying **free thyroxine** versus total thyroxine is crucial for accuracy.
- **Free T4** levels, along with TSH, are generally the most informative initial tests for diagnosing conditions like hyperthyroidism.
Question 29: An investigator is studying the crossbridge cycle of muscle contraction. Tissue from the biceps brachii muscle is obtained at the autopsy of an 87-year-old man. Investigation of the muscle tissue shows myosin heads attached to actin filaments. Binding of myosin heads to which of the following elements would most likely cause detachment of myosin from actin filaments?
A. ATP (Correct Answer)
B. Troponin C
C. Tropomyosin
D. ADP
E. cGMP
Explanation: ***ATP***
- The binding of **ATP** to the **myosin head** causes a conformational change that reduces its affinity for actin, leading to detachment.
- This step is crucial for the muscle to relax and for the subsequent power stroke to occur.
*Troponin C*
- **Troponin C** is a regulatory protein that binds calcium, which then causes a conformational change in the troponin-tropomyosin complex, revealing the **actin binding sites** for myosin.
- It does not directly cause myosin detachment; instead, it facilitates the binding of myosin to actin.
*Tropomyosin*
- **Tropomyosin** is a long, fibrous protein that covers the **myosin-binding sites** on actin in a relaxed muscle, preventing cross-bridge formation.
- Its movement, regulated by troponin, allows myosin to bind, but it does not directly cause detachment.
*ADP*
- **ADP** is released from the myosin head during the power stroke, but its binding does not cause detachment; rather, it is present during the strongly bound state before **ATP** binds.
- The presence of **ADP** and inorganic phosphate (Pi) often promotes the strong binding of myosin to actin.
*cGMP*
- **cGMP** (cyclic guanosine monophosphate) is a second messenger involved in various cellular processes, including smooth muscle relaxation, but it is not directly involved in the cross-bridge cycle and detachment of **myosin from actin** in skeletal muscle.
- Its primary role in muscle physiology is often linked to nitric oxide signaling and vasodilation.
Question 30: An investigator is working with a mutant strain of mice that lack a consistent density of sarcolemmal transverse tubules in the skeletal muscle cells. Which of the following is the most likely associated finding as a result of this abnormality?
A. Impaired binding of acetylcholine to nicotinic acetylcholine receptors
B. Decreased entry of calcium at the presynaptic membrane
C. Impaired synchronization of cross-bridge formation (Correct Answer)
D. Increased activation of myosin-light-chain phosphatase
E. Decreased expression of sarcolemmal Na+/K+ ATPase
Explanation: ***Impaired synchronization of cross-bridge formation***
- **Transverse tubules (T-tubules)** are crucial for rapidly propagating the **action potential** from the sarcolemma deep into the muscle fiber, ensuring a uniform and synchronized release of calcium from the sarcoplasmic reticulum.
- A lack of consistent T-tubule density would lead to **desynchronized calcium release** and thus uncoordinated activation of **myosin-actin cross-bridges**, resulting in inefficient and impaired muscle contraction.
*Impaired binding of acetylcholine to nicotinic acetylcholine receptors*
- The binding of **acetylcholine (ACh)** to **nicotinic ACh receptors** occurs at the **neuromuscular junction** on the surface of the sarcolemma, prior to the generation of the action potential that travels down the T-tubules.
- T-tubule abnormalities primarily affect the propagation of the electrical signal *after* it has been initiated on the sarcolemma, not the initial binding of neurotransmitters.
*Decreased entry of calcium at the presynaptic membrane*
- **Calcium entry at the presynaptic membrane** is vital for the release of **acetylcholine** into the synaptic cleft.
- This process is upstream of muscle cell excitation and T-tubule function; T-tubule abnormalities in the muscle cell would not affect calcium entry into the **neuron's presynaptic terminal**.
*Increased activation of myosin-light-chain phosphatase*
- **Myosin light chain phosphatase (MLCP)** plays a significant role in **smooth muscle relaxation** by dephosphorylating myosin light chains, leading to detachment of cross-bridges.
- In **skeletal muscle**, calcium binding to **troponin C** initiates contraction by shifting tropomyosin, allowing myosin to bind to actin; MLCP is not the primary regulator of contraction/relaxation in skeletal muscle.
*Decreased expression of sarcolemmal Na+/K+ ATPase*
- The **Na+/K+-ATPase** is essential for maintaining the **resting membrane potential** of the muscle cell and restoring ion gradients after an action potential.
- While important for muscle function, its expression would not be directly or primarily affected by a structural defect in the **T-tubule system**, which is mainly involved in action potential propagation and calcium release.
