A study is evaluating fluoride absorption. Which factor could most significantly affect the accuracy of the results?

Correct answer: C. Altitude and its influence on fluoride absorption

• A. ❌ Instrument sensitivity affects precision/reliability but is a measurement issue, not a study accuracy/validity threat.
• B. ❌ Variation in fluoride concentration is a baseline variable that would be controlled by specifying the dose.
• C. ✅ Correct. Altitude affects physiology (respiration, blood pH, renal function) in ways that could alter fluoride absorption, acting as an uncontrolled confounding variable threatening internal validity.
• D. ❌ Sex differences are generally controlled for through study design (stratification/randomization).