Biology Biomolecules NUMS 2022
PMDC Verified Question 78 of 142
Which of the following is not a carbohydrate?
A
Sucrose (C12H22O11)
B
Lactic acid (C3H6O3)
C
Glucose (C6H12O6)
D
Rhamnose (C6H12O5)
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option B: Lactic acid (C3H6O3)
Concept:

Not all molecules that loosely fit the empirical formula of a carbohydrate are biologically classified as carbohydrates.

Solution:

  • Lactic acid has the formula \( C_3H_6O_3 \), which mathematically fits the \( C_n(H_2O)_n \) ratio.


  • However, structurally, it contains a carboxylic acid group (-COOH) and a single hydroxyl group, not the polyhydroxy aldehyde/ketone structure required to be a sugar.


  • It is a metabolic by-product of anaerobic respiration, completely distinct from carbohydrates.


Why other options are incorrect:

  • Glucose & Sucrose: Standard textbook carbohydrates.


  • Rhamnose: A naturally occurring deoxy sugar (a true carbohydrate, despite deviating from the strict \(1:2:1\) water ratio).

Quality & Fidelity Assurance: Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.

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