Muscle physiology: energy metabolism: practice question

A Physiology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Physiology · Muscle physiology: energy metabolism

Sample item

A 19-year-old man has painful cramps and dark urine after sprinting but feels better after a brief rest (second wind). In an ischaemic forearm exercise test, venous ammonia rises but lactate does not. What is the physiological defect?

AFailure of acetylcholine release at the neuromuscular junctionBDefective calcium re-uptake by the sarcoplasmic reticulumCInability to mobilise muscle glycogen, so anaerobic glycolysis yields no lactateDInability to oxidise long-chain fatty acids in mitochondria

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

C. Inability to mobilise muscle glycogen, so anaerobic glycolysis yields no lactate

In McArdle disease, deficiency of muscle glycogen phosphorylase prevents glycogen breakdown, so ischaemic exercise cannot generate lactate by glycolysis. Muscle uses ATP without regeneration, leading to cramps and rhabdomyolysis with myoglobinuria, and ammonia rises as adenylate is deaminated. The second-wind phenomenon occurs as blood-borne glucose and fatty acids become available.

Option by option

A. Failure of acetylcholine release at the neuromuscular junction
Failure of release at the junction would cause fatigable weakness with a decrement, not metabolic findings.
B. Defective calcium re-uptake by the sarcoplasmic reticulum
Defective calcium re-uptake causes delayed relaxation, as in Brody disease, with normal lactate.
C. Inability to mobilise muscle glycogen, so anaerobic glycolysis yields no lactate (correct)
Absent glycogenolysis explains the flat lactate response with a normal ammonia rise.
D. Inability to oxidise long-chain fatty acids in mitochondria
Defective fatty acid oxidation causes exercise intolerance after prolonged exercise or fasting, with a normal lactate rise.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is C, Inability to mobilise muscle glycogen, so anaerobic glycolysis yields no lactate. In McArdle disease, deficiency of muscle glycogen phosphorylase prevents glycogen breakdown, so ischaemic exercise cannot generate lactate by glycolysis.

Why is "Failure of acetylcholine release at the neuromuscular junction" not the answer?

Failure of release at the junction would cause fatigable weakness with a decrement, not metabolic findings.

Why is "Defective calcium re-uptake by the sarcoplasmic reticulum" not the answer?

Defective calcium re-uptake causes delayed relaxation, as in Brody disease, with normal lactate.

Why is "Inability to oxidise long-chain fatty acids in mitochondria" not the answer?

Defective fatty acid oxidation causes exercise intolerance after prolonged exercise or fasting, with a normal lactate rise.

What concept does this question test?

It tests Muscle physiology: energy metabolism in Physiology. Muscle uses ATP without regeneration, leading to cramps and rhabdomyolysis with myoglobinuria, and ammonia rises as adenylate is deaminated. The second-wind phenomenon occurs as blood-borne glucose and fatty acids become available.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Muscle physiology: energy metabolism?

For each system, sketch the key graph (pressure–volume loop, oxygen dissociation curve, nephron handling) and explain it aloud. Then use the flashcards and the other Physiology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.