During intense physical exercise, accumulation of which one of the following substances in the muscles of a person leads to painful muscle cramps?

Introduction / Context:
When a person performs vigorous physical exercise, their muscles require more energy and oxygen. If the oxygen supply becomes insufficient, the muscles switch partly to anaerobic respiration. This can lead to the build up of certain metabolites that are associated with fatigue and cramps. This question asks which specific substance accumulates in muscles and is commonly linked with muscle cramps during or after heavy exercise.

Given Data / Assumptions:

• The situation is intense muscular activity such as sprinting or heavy exercise.
• The options include lactic acid, ethanol, pyruvic acid, and glucose.
• We assume typical human muscle metabolism under low oxygen conditions.

Concept / Approach:
Under normal aerobic conditions, glucose is broken down into pyruvic acid, which then enters the mitochondria and is further oxidized in the Krebs cycle. When oxygen is limited, pyruvic acid is converted to lactic acid in muscle cells via anaerobic respiration (lactic acid fermentation). Lactic acid accumulation is associated with decreased pH, muscle fatigue, and cramp like sensations. Ethanol production occurs in some microorganisms, not in human muscles. Pyruvic acid itself is an intermediate that is usually further metabolized, and glucose is the initial fuel, not the direct cause of cramp when accumulated.

Step-by-Step Solution:
Step 1: Evaluate option A, lactic acid. This is produced in significant amounts during anaerobic respiration in muscles and is known to accumulate during intense exercise.Step 2: Evaluate option B, ethanol. Ethanol is produced by yeast and some bacteria during fermentation; human muscles do not produce ethanol during exercise.Step 3: Evaluate option C, pyruvic acid. While pyruvic acid is formed during glycolysis, it usually does not accumulate heavily; it is either oxidized or converted into lactic acid.Step 4: Evaluate option D, glucose. Glucose is a fuel source and is broken down during exercise; its accumulation is not the direct reason for cramps.Step 5: Conclude that lactic acid is the correct answer.

Verification / Alternative check:
A simple verification is to recall that textbooks often mention lactic acid accumulation as the cause of muscle soreness and cramps after strenuous exercise. Cooling down and stretching after exercise help to remove lactic acid from muscles more gradually, which aligns with this explanation. No standard physiology text claims that ethanol or glucose buildup in muscles is responsible for cramps.

Why Other Options Are Wrong:
Option B is wrong because ethanol is not produced by human muscle cells under exercise conditions. Option C is wrong because pyruvic acid is typically converted into other products; the discomfort is more directly associated with lactic acid. Option D is wrong because glucose is consumed rather than accumulated during intensive exercise and does not directly cause cramps when present in high concentration.

Common Pitfalls:
Students may confuse pyruvic acid and lactic acid since both are central to glycolysis and anaerobic pathways. Remember that lactic acid is the end product of anaerobic glycolysis in human muscles. Another pitfall is assuming that any metabolic intermediate could be responsible for cramps; in exams, lactic acid is the specific metabolite linked to this symptom during heavy exercise.

Final Answer:
Lactic acid.