MDCAT common mistakes
MDCAT · Understand the traps and how to avoid them.
In genetics questions, writing a phenotype ratio from a remembered Punnett-square pattern before confirming the parental genotypes and whether the cross is monohybrid, dihybrid, or a test cross.
Familiar ratios such as 3:1 and 9:3:3:1 are recognised quickly, so candidates match the question to the nearest memorised pattern instead of deriving the cross from the stated genotypes.
What to do instead
Before calculating, write each parent's genotype, gametes, and the phenotype associated with each genotype. If a test cross is mentioned, explicitly set the unknown parent against the homozygous recessive and derive the ratio from that cross.
In physiology and enzyme questions, choosing an answer that names the correct substance but places it in the wrong compartment or gives the wrong direction of flow, such as putting a glycolysis product in the mitochondrion or reversing a hormone's source and target.
Metabolic pathways and hormone actions are often memorised as word lists rather than as spatial maps, so the substance is recognised but its location or direction is not checked.
What to do instead
Redraw each pathway as a compartment map with arrows: cytoplasm, mitochondrial matrix, inner membrane, bloodstream, and target cell. Label each arrow with the reactant, product, enzyme or hormone, and direction, then answer only after locating the item on the map.
In stoichiometry, calculating product mass directly from the first reactant mentioned, without converting reactants to moles and without identifying the limiting reagent.
The familiar sequence mass = moles × molar mass is applied too early, and the first number in the question feels like the natural starting point.
What to do instead
Use a three-line mole map: convert every reactant from grams to moles, divide each by its balanced-equation coefficient, and select the smallest quotient as the limiting reagent. Calculate product moles from that reactant only, then convert to mass.
In equilibrium and acid-base items, substituting concentrations into a K or Ka expression without first balancing the equation, adding phase labels, or checking whether the values are initial or equilibrium concentrations.
Candidates recognise the equilibrium-expression formula and substitute the numbers that look like concentrations, skipping the conditions under which the expression is valid.
What to do instead
Write the balanced equation with phase labels, write the K expression from that equation, and mark each given value as initial or equilibrium. If values are initial, set up an ICE table before substituting; if a species is a pure solid or liquid, omit it from the expression.
In physics, selecting a kinematics or dynamics equation because it contains the symbol asked for, without checking whether acceleration is constant or whether the motion is rotational rather than translational.
Formula-sheet matching is faster than condition checking, so equations are treated as universal rather than as models with stated assumptions.
What to do instead
Before substitution, write the knowns, the unknown, and a condition checklist: Is acceleration constant? Is the net force central? Is the system rotating? Is the collision elastic? Use only equations whose assumptions match the checklist.
Dropping signs on vector quantities such as displacement, velocity, acceleration, force, and electric field, then adding magnitudes as if the quantities were scalars.
Everyday arithmetic treats magnitude as the whole quantity, and sign errors are easy to miss when the final number looks reasonable.
What to do instead
Define a positive direction at the start, draw an arrow for each vector, and carry signs through every equation. At the end, state the direction separately and check that the sign of the answer agrees with the physical situation.
In reading comprehension, choosing an option because it repeats words from the passage rather than because it answers the specific question stem.
Lexical overlap creates a feeling of familiarity, and under time pressure that feeling is mistaken for textual support.
What to do instead
Paraphrase the question stem in your own words before looking at the options. For each option, locate the sentence that supports it and check whether that sentence answers the stem's exact scope; eliminate options supported only by repeated vocabulary.
In grammar items, matching the verb to the noun nearest to it in a subject-verb agreement sentence that contains an interrupting phrase such as 'along with' or 'as well as'.
Proximity concord is common in speech, so the ear accepts the nearest noun as the subject even when the grammatical head noun is earlier and different in number.
What to do instead
Bracket or mentally delete prepositional phrases, relative clauses, and parenthetical insertions. Identify the head noun of the subject, decide its number, then make the verb agree with that head noun rather than with the nearest noun.
In logical reasoning, accepting a conclusion as valid because it is plausible or consistent with the premises, without checking whether it must follow from them.
Everyday reasoning treats strong support and entailment as the same thing, so a believable conclusion feels logically forced.
What to do instead
Apply the validity test: ask whether the premises could all be true while the conclusion is false. If yes, the conclusion does not follow. For assumption questions, use the negation test: negate the candidate assumption and check whether the argument collapses.
In logical reasoning and English items, skimming past qualifiers and exception words such as 'except', 'not', 'only', 'most', 'some', and 'all' before selecting an answer.
These short words are visually low-salience, and the mind often completes the question according to the expected pattern rather than the printed wording.
What to do instead
Underline every quantifier and negation in the stem and in each option. Restate the task in your own words, including the qualifier, before evaluating options, and reject any option whose scope is stronger or weaker than the stem allows.