USJ Medical Sciences Concours - Medicine, Dental Medicine & Pharmacy common mistakes
USJ Medical Sciences Concours - Medicine, Dental Medicine & Pharmacy · Understand the traps and how to avoid them.
Compares two saline/glucose solutions by molarity alone - for example, calls 0.15 mol/L NaCl and 0.15 mol/L CaCl2 'the same concentration, so both isotonic' - and therefore predicts no volume change for a red cell in the CaCl2 solution.
The candidate equates concentration (mol/L of formula units) with osmolarity (mol/L of dissolved particles), forgetting that each dissolved salt dissociates into a fixed number of particles and that only non-penetrating solutes determine tonicity.
What to do instead
Convert every compartment to osmolarity before reasoning: osmolarity = sum over solutes of (i x C), with i = 2 for NaCl and 3 for CaCl2, so 0.15 mol/L CaCl2 is about 450 mOsm, hypertonic to plasma (~290 mOsm), and the cell crenates. Write the number next to each solution, compare with 290 mOsm, then state the direction of water movement (toward the higher osmolarity) and the volume outcome.
Classifies the pre-ovulatory LH surge or oxytocin-driven uterine contraction as negative feedback, justifying it with 'the hormone rises and then falls again'.
The candidate judges feedback type from the shape of a hormone curve or from a memorised list of hormones, instead of asking whether the response opposes or amplifies the original deviation from the set point.
What to do instead
For each loop, write out five things: set point, sensor, integrator, effector, response. Then ask one question - does the response reduce the starting deviation (negative) or increase it until an external event terminates it (positive)? Apply it: uterine stretch increases oxytocin, which increases contraction, which increases stretch (positive, ending at delivery); oestrogen is inhibitory at low levels but flips to positive feedback above a threshold, producing the LH surge.
Calculates the pH of a buffer after adding strong acid by subtracting the added moles from the conjugate base only, or by substituting the original molarities into the Henderson-Hasselbalch equation and ignoring the added volume.
The candidate treats Henderson-Hasselbalch as a formula about starting concentrations rather than about the equilibrium amounts actually present, and works in moles while the formula needs concentrations, so the dilution step gets dropped.
What to do instead
Do a mole table first: n(A-) minus n(H+), n(HA) plus n(H+), for a strong acid added; then divide both by the new total volume and apply pH = pKa + log([A-]/[HA]). Check buffer capacity: if the added strong acid exceeds n(A-), the buffer is exhausted and the pH must be computed from the excess strong acid. Note also that in the ratio n(A-)/n(HA) the volume cancels, which is why the mole-only shortcut works - but only if both species are in the same volume.
Assigns R/S to a chiral centre without re-orienting when the lowest-priority group points toward the viewer, and predicts a single inverted product for an SN1 substitution at a secondary chiral carbon.
CIP priorities are learned as a text rule rather than as a spatial operation, so the 180-degree flip implied by a wedge/dash is never performed; and 'inversion means substitution' gets over-generalised from SN2 to all substitution reactions.
What to do instead
Use the swap method every time: swap the lowest-priority group with the group pointing away from you, assign the configuration from the remaining three, then invert the answer because you made one swap. For mechanism, decide first whether a carbocation intermediate is plausible (secondary/tertiary, weak nucleophile, protic solvent): SN1 goes through a planar carbocation, giving racemisation with slight net inversion; SN2 gives clean inversion. State the mechanistic reason, not just the stereochemical label.
Substitutes radius in cm, pressure drop in mmHg and flow in mL/min directly into Poiseuille's law, and concludes that a 20% narrowing of a vessel reduces flow by about 20%.
Clinical physiology routinely quotes pressures in mmHg and flows in mL/min, which makes direct substitution feel legitimate; and the fourth-power dependence of flow on radius is stated in the course but rarely practised numerically.
What to do instead
Either convert everything to SI (r in m, dP in Pa, viscosity in Pa·s, Q in m³/s) or use the ratio form, which needs no unit conversion: Q2/Q1 = (r2/r1)^4 x (dP2/dP1) x (L1/L2) x (eta1/eta2). Practise the fourth power mentally: 0.8^4 = 0.41, so a 20% radius reduction cuts flow to about 41% if pressure and length are unchanged - a 59% fall, not 20%.
Applies 1/R = 1/R1 + 1/R2 to two resistors that are in parallel with each other but in series with a third, and computes the battery current as EMF divided by the external resistance only, ignoring internal resistance.
The candidate pattern-matches on the phrase 'two resistors' without redrawing the circuit, and treats the EMF as if it were the terminal voltage, which is only true for an ideal source.
What to do instead
Redraw the circuit on the answer sheet, label nodes with letters, and collapse from the innermost parallel block outward before touching Ohm's law. Always include the internal resistance r, use terminal voltage V = EMF - I r, and cross-check the result with Kirchhoff's loop rule plus a power balance (EMF x I = I² x R_total, including r).
Given a test with 95% sensitivity and 90% specificity for a disease with 1% prevalence, answers that a patient with a positive result has 'about a 95% chance' of having the disease.
The conditional probability is read in the wrong direction - P(positive | disease) is silently replaced by P(disease | positive) - and the base rate is treated as irrelevant because it never appears in the sensitivity/specificity sentence.
What to do instead
Never compute this one abstractly. Build a 2x2 table on 10,000 people: 100 diseased, of whom 95 test positive; 9,900 healthy, of whom 990 falsely test positive. Then PPV = 95/(95 + 990), about 8.8%. Before any such question, write the requested quantity with an explicit vertical bar, and note that a rare disease makes a positive result mostly false positives even with an excellent test.
Reads a semi-logarithmic plot (log concentration against time) as if concentration fell linearly, taking the half-life from the slope value directly; or solves an exponential equation such as 2^x = 8 by dividing by 2.
Linear intuition is applied to a process whose change is multiplicative, and the logarithm laws (log ab = log a + log b, log a^n = n log a, base-10 versus natural log conversion) are not fluent enough to be used under time pressure.
What to do instead
First label the axes explicitly: if the y-axis is log concentration and the plot is a straight line, the decay is first-order and the slope equals -k/2.303 for log10 or -k for ln. Convert with ln x = 2.303 log10 x. For exponential equations, take the log of both sides rather than dividing: 2^x = 8 gives x log 2 = log 8, so x = 3. Sanity-check by substitution.
Answers a data-interpretation item using the textbook mechanism even though the described experiment shows no difference between the treated and control groups, or infers causation from an observational cohort study.
Most of the exam rewards recall, so recall is the default strategy; the candidate also lacks a habitual checklist for study design, so the presence of a control group, randomisation or blinding is not examined before choosing.
What to do instead
Restate the claim in the stem, then extract only the numbers and comparisons actually given, and ask what the design licenses: was there a concurrent control, was allocation randomised, was measurement blinded, is there an identified confounder? Treat any option containing 'proves', 'causes' or 'demonstrates that X produces Y' as suspect in observational data. Where stem data and textbook mechanism disagree, the stem wins.
Chooses the option that repeats a phrase verbatim from the passage, without noticing that the stem asks which statement is NOT supported, or what the author's main purpose is; and answers from personal scientific knowledge when the passage says otherwise.
Keyword matching feels fast and safe under time pressure, and negation words and scope qualifiers (mainly, primarily, in vitro, according to the passage) are skipped during a first skim.
What to do instead
Read the stem before the passage and circle the task word - NOT, EXCEPT, mainly, according to the author. Then locate and bracket the single sentence or two that answers it, paraphrase that text in your own words, and only then look at the options. Eliminate any option whose support you cannot point to in the bracketed text, even if it is scientifically true.