AAMC Full Lengths error log

I missed a question involving NADH and electron transport. What organic transformation should I associate with NADH/NAD⁺?

NADH is a reducing agent and becomes oxidized to NAD⁺.

Alcohol ⇌ Carbonyl

• NAD⁺ → NADH = carbonyl → alcohol (reduction)

• NADH → NAD⁺ = alcohol → carbonyl (oxidation)

I missed a question asking for the component found in FAD, FMN, and riboflavin. What structure should I recognize?

FAD, FMN, and riboflavin are flavin cofactors.

They all contain an isoalloxazine ring, the redox-active flavin structure.

Shortcut:

• Adenine = part of FAD only

• Flavin/isoalloxazine = FAD, FMN, riboflavin

• Ubiquinone = quinone + long hydrophobic tail

• Histidine = imidazole side chain

I missed a question asking why an exergonic electron transport reaction does not release much heat. What should I think when a gradient is formed?

The energy is being stored in an electrochemical gradient, not released as heat.

MCAT rule:

Exergonic reaction + active transport = energy coupling

Examples:

• ETC → proton gradient

• Na⁺-NQR → sodium gradient

• ATP hydrolysis → ion pumping

Enzymes change activation energy, not ΔG. 🚨 Common trap.

I missed a question asking for the molecular formula of pyrrole. What should I know?

Pyrrole is:

• A 5-membered aromatic ring

• 4 carbons + 1 nitrogen

• Lone pair participates in aromaticity

Pyridine = 6-membered aromatic N ring

I missed a protein digestion + SDS-PAGE question. What happens to bands over time during proteolysis?

Proteolysis cuts a large protein into smaller fragments.

On SDS-PAGE:

• Larger proteins stay near the top

• Smaller fragments travel farther down

• Over time: intact band fades, smaller lower bands increase

Digestion = smaller pieces = lower bands.

I missed a question asking about the precursor of progesterone. What should I remember about steroid hormone synthesis

All steroid hormones are derived from cholesterol.

Do not confuse with:

• Tyrosine → catecholamines

• Tryptophan → serotonin/melatonin

• Histidine → histamine

Fusion = overcome electrostatic repulsion to reach strong-force range

I missed a question asking which residue could be selectively modified. How do I identify selective modification?

Selective modification means:

One residue is modified while others are not.

Look for a condition where:

• Target residue = modified

• All other residues = 0% (or minimal modification)

I missed a question because I focused on Trp instead of the residues interacting with Trp. What heterocycle is found in histidine?

Histidine contains an imidazole ring.

Key amino acid heterocycles:

• Histidine → Imidazole

• Tryptophan → Indole

• Proline → Pyrrolidine

Electron-withdrawing groups (Cl, F, NO₂, carbonyls) increase acidity when nearby.

MCAT acidity trend:

\text{CCl}_3\text{COOH} > \text{CH}_2\text{ClCOOH} > \text{CH}_3\text{COOH}

More electron withdrawal → more stable conjugate base → lower pKa.

I missed a deprotonation-order question with two carboxylic acids. How do I decide which deprotonates first?

The most acidic proton (lowest pKa) is removed first.

Look for electron-withdrawing groups near the acidic proton.

More EWG → more stable conjugate base → lower pKa → deprotonates first.

Atwood machine formula

I missed a CD spectroscopy question. What feature of proteins gives rise to circular dichroism?

CD spectroscopy detects chirality.

In proteins, chirality primarily arises from the α-carbons of amino acids.

Most amino acids have a chiral α-carbon attached to:

• NH₂

• COOH

• H

• R group

MCAT shortcut:
CD spectroscopy → chirality → α-carbon.

I missed a mutation question involving Leu → Ser. What property remains unchanged?

Leucine:

• Nonpolar

• Hydrophobic

• Neutral

Serine:

• Polar

• Can H-bond

• Neutral

Changes:

• Molecular weight ✓

• Hydrophobicity ✓

• H-bonding ✓

Unchanged:

• Net charge (0)

I missed a chromatography question involving addition of a carboxylic acid group. How do I choose the separation method?

First ask:

What property changed?

Adding a carboxylic acid:

• At pH 8 → COO⁻

• Protein becomes more negatively charged

Use:

• Anion-exchange chromatography for negatively charged proteins

MCAT chromatography map:

• Charge → Ion exchange

• Size → Size exclusion

• Binding affinity → Affinity chromatography

• Polarity → TLC/HPLC

I missed a table interpretation question. What should I do before using outside biology knowledge?

For table questions:

1. Read the question.

2. Look only at the relevant columns.

3. Identify substrate and product.

4. Match directly to answer choices.

MCAT rule:

If the answer is in the table, don’t invent a harder explanation.

What makes a membrane transporter electrogenic?

A transporter is electrogenic if it produces a net movement of charge across the membrane.

I missed a passage question asking about the benefit of a modified protein therapy. How should I reason through it?

Identify:

1. Desired effect

2. Undesired effect

Choose the answer that:

• Keeps the desired effect

• Removes the consequence of the undesired effect

EPO example:

• Desired: prevent neuronal apoptosis

• Undesired: increase RBC production

Benefit:
→ Neuroprotection without increased blood viscosity.

Why do some human proteins expressed in E. coli lose biological activity?

E. coli lacks many eukaryotic post-translational modifications.

Most important:

Glycosylation

If a protein is a glycoprotein:

• Amino acid sequence may be correct

• Carbohydrate chains may be missing

• Protein may be nonfunctional

MCAT shortcut:
Human glycoprotein + E. coli = think glycosylation problem.

A dominant mutation causes increased physiological activity. What type of mutation should I suspect?

Usually a gain-of-function mutation.

Examples:

• Increased ligand affinity

• Increased protein expression

• Constitutive receptor activation

• Increased signaling

MCAT rule:

More phenotype → more pathway activity → gain-of-function.

How do I identify evidence that invalidates a scientific model?

1. Identify the model’s central prediction.

2. Find the observation that directly contradicts that prediction.

Unit Membrane Model:

• Proteins remain on membrane surfaces.

• Proteins do not penetrate the bilayer.

Observation:

• Proteins inside the hydrophobic region

→ Invalidates the model.

MCAT shortcut:
“Invalidate” = “contradict a required prediction.”

If Cxcl1 is lower in Ogg1-/- mice, that means normal OGG1 must stimulate TNFα-induced Cxcl1 expression.

So statement II is true: OGG1 stimulates the TNFα induced Cxcl1 expression.

-/- means absence, so wt has OGG1 normal

How do I identify the independent variable in an experiment?

The IV is what the researchers manipulate or assign.

The DV is what the researchers measure.

Example:
Researchers create dating profiles that are:

• Similar to actual self

• Similar to ideal self

IV = profile similarity

DV = attraction to the date

A person stops attending events because attending causes feelings of inferiority. What operant conditioning principle is occurring?

Behavior:

• Attending events ↓

Consequence:

• Unpleasant feeling added

Result:

• Positive punishment

Rule:

Positive = added

Negative = removed

Reinforcement = behavior increases

Punishment = behavior decreases

MCAT shortcut:
If behavior decreases, think punishment first.

When you see:

sociological conceptualization

immediately think:

• institutions

• social structures

• social construction

• norms

• stratification

How do I approach MCAT questions that ask about results from multiple studies?

1. Summarize each study in one sentence.

2. Identify the main finding of each.

3. Look for the answer that combines both findings.

Which theories are most similar to the life course perspective?

Life course perspective:

• Early experiences affect later outcomes

• Cumulative risk

• Protective factors

• Development over time

Most similar:

• Potentiator model (risk accumulation)

• Protective model (buffering effects)

MCAT shortcut:

Life course = risk factors + protective factors across development.