Cover: Clue, coenzyme, enzyme: one method for eight vitamins.
Why does a confused man on the ward get thiamine before his dextrose drip? Why does a tuberculosis patient on isoniazid need a second vitamin? And why do two vitamins that cause the same anaemia need two different blood tests to tell them apart?
Last updated: October 22, 2026
Water-soluble vitamin questions in NEET PG almost always turn on one clue: a sign, a drug, a diet or a lab value. Link that clue to the vitamin's coenzyme and the enzyme it serves, and the answer is usually one step away.
The National Eligibility cum Entrance Test – Postgraduate (NEET PG), the Institute of National Importance Combined Entrance Test (INI-CET) and the Foreign Medical Graduate Examination (FMGE) all test this chapter. The questions rarely ask "what is the RDA". They ask what a patient's clue means.
The coenzyme and enzyme facts here follow Harper's Illustrated Biochemistry, Lippincott Illustrated Reviews: Biochemistry, and two Indian texts: Biochemistry by U Satyanarayana and Textbook of Biochemistry for Medical Students by DM Vasudevan. The numbers come from the NIH Office of Dietary Supplements (ODS) fact sheets and three journal reviews, each named where it is used. Where two sources give different figures, you get both.
Quick facts (all sourced below)
- B1 (thiamine) works as thiamine pyrophosphate (TPP). Up to 80% of people with chronic alcoholism develop thiamine deficiency (NIH ODS).
- The full Wernicke triad appears in only about 16% of patients (figure cited in Li and Xing, 2025).
- Pellagra has four favourite exam causes: three non-diet (Hartnup disease, carcinoid syndrome, isoniazid) and one diet (jowar, through excess leucine).
- Homocysteine rises in both B12 and folate deficiency. Methylmalonic acid (MMA) is typically normal in folate deficiency (Bedz and Forsyth, 2026).
- Scurvy signs can appear within 1 month of taking in less than about 10 mg/day of vitamin C (NIH ODS).
How Do You Turn a Vitamin Stem Into an Answer?
Use a three-step chain: clue, coenzyme, enzyme. The clue names the system that failed. The coenzyme names the vitamin. The enzyme explains the sign, and that is what the options test.
Here is the chain for the whole chapter. Learn the middle two columns cold; the clues then sort themselves.
| Vitamin | Active coenzyme | Enzyme the exam asks about | Classic clue |
|---|---|---|---|
| B1 thiamine | TPP | Pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase, transketolase | Alcohol use, ophthalmoplegia, high-output heart failure |
| B2 riboflavin | FAD, FMN | Flavoprotein dehydrogenases; glutathione reductase | Angular stomatitis, cheilosis, magenta tongue |
| B3 niacin | NAD, NADP | Dehydrogenases (redox) | Rash on sun-exposed skin, diarrhoea, dementia |
| B6 pyridoxine | PLP | Transaminases, ALA synthase, glutamate decarboxylase | Isoniazid, convulsions, sideroblastic anaemia |
| B7 biotin | Biotin (on the enzyme) | Carboxylases | Raw egg whites |
| B9 folate | Tetrahydrofolate | Thymidylate synthesis; methionine synthase (with B12) | Megaloblastic anaemia, neural tube defects |
| B12 cobalamin | Methylcobalamin, adenosylcobalamin | Methionine synthase; methylmalonyl-CoA mutase | Megaloblastic anaemia plus spinal cord signs |
| C ascorbic acid | Ascorbate | Prolyl and lysyl hydroxylases | Bleeding gums, perifollicular bleeding |
This is the same habit that sorts the glycogen storage disease stems: name the enzyme, then predict the sign. Do it for vitamins and the "which deficiency" stem stops being a memory test.
Caption: Clue, coenzyme, enzyme: the three columns that answer most vitamin stems.
Clue 1: Alcohol, Confusion and Stuck Eyes: Why Is Thiamine Given Before Glucose?
Thiamine (B1) works as TPP, a coenzyme for pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase and transketolase. A glucose load pushes more pyruvate through these enzymes and uses up what little thiamine is left, so thiamine goes first.
The NIH ODS thiamin fact sheet says about 80% of body thiamine sits as thiamine diphosphate, the same molecule as TPP. It counts five TPP-dependent enzymes. The exam usually wants three of them: the two dehydrogenases in energy metabolism and transketolase in the pentose phosphate pathway.
Beriberi comes in two forms. Dry beriberi is a nerve problem: peripheral neuropathy and wasting. Wet beriberi is a heart problem: high-output cardiac failure with oedema. The ODS sheet describes the same split, calling heart failure the rarer outcome. If the stem says "oedema and a bounding pulse in a rice-eating labourer", think wet.
Wernicke encephalopathy is the acute brain form. The textbook triad is confusion, ophthalmoplegia and ataxia. A 2025 review by Siping Li and Chengzhi Xing in Frontiers in Neurology cites the trap from earlier studies: the full triad appears in only 16% of patients. Their cited figures are about 82% for encephalopathy, 29% for eye signs and 23% for ataxia, and 19% of patients show none of the three. So a stem with only confusion in a person who drinks heavily can still be Wernicke.
Li (Department of Ultrasound Medicine) and Xing (Department of Neurology), both at Pu'er People's Hospital, China, cite the European Federation of Neurological Societies (EFNS) on the drip question: "glucose should be prohibited before supplementing vitamin B1, as glucose metabolism continues to consume the deficient vitamin B1, exacerbating the condition." Harper's gives the mechanism: without TPP, pyruvate cannot enter the citric acid cycle, so it piles up as lactate.
Korsakoff syndrome is the chronic stage. The ODS sheet lists severe short-term memory loss, disorientation and confabulation. It adds that without treatment up to 20% of people with Wernicke encephalopathy die. The classic status test is the erythrocyte transketolase assay. ODS gives the "TPP effect" bands: 0%–15% in healthy people, 15%–25% in marginal deficiency and above 25% in deficiency.
Caption: The triad is a textbook picture. In patients, most show one or two parts of it.
Clue 2: Cracked Lip Corners and a Magenta Tongue: Is It B2 or B6?
A magenta tongue points to riboflavin (B2), which works as FAD and FMN. Angular stomatitis and cheilosis occur in both B2 and B6 deficiency, so the mouth alone does not settle it. Look for the second clue.
The NIH ODS riboflavin fact sheet lists the signs of ariboflavinosis: angular stomatitis, cheilosis, swelling and redness of the mouth and throat, sore throat, itchy red eyes and hair loss. Satyanarayana and Vasudevan add the magenta tongue and corneal vascularisation as the classic exam pairing.
The trap is overlap. The ODS vitamin B6 sheet lists cheilosis and glossitis too.
So read the rest of the stem. A tuberculosis drug or a convulsing infant points to B6. Isolated mouth and eye signs in someone who avoids milk point to B2. The ODS sheet notes that people who take little milk or other dairy foods are at risk of low riboflavin.
The status test is the erythrocyte glutathione reductase activity coefficient (EGRAC), because glutathione reductase is an FAD enzyme. ODS uses 1.2 or less as adequate, 1.2–1.4 as marginal and above 1.4 as deficient, while noting that the best thresholds are uncertain.
Clue 3: Rash on Sun-Exposed Skin and Diarrhoea: Which Cause of Pellagra?
Pellagra is niacin (B3) deficiency, and niacin works as NAD and NADP. The exam rarely stops at "pellagra". It asks why the patient ran out, and the four favourite answers are Hartnup disease, carcinoid syndrome, isoniazid and a jowar diet.
The textbook "3 Ds" are dermatitis, diarrhoea and dementia; the fourth D is death. The NIH ODS niacin fact sheet describes a pigmented rash on sun-exposed skin, a bright red tongue, diarrhoea or constipation, depression, memory loss and hallucinations, and death if untreated. A rash around the neck is called Casal's necklace.
The first cause is still a poor diet: ODS says pellagra today is mostly limited to people living in poverty on very limited diets. The second key fact is that the body can make niacin from tryptophan. ODS counts 60 mg of tryptophan as worth 1 mg of niacin. So anything that steals tryptophan, or blocks the conversion, can cause pellagra:
- Hartnup disease. A transport defect means tryptophan is poorly absorbed in the gut and lost in the urine (ODS).
- Carcinoid syndrome. The tumour turns tryptophan into serotonin instead of niacin (ODS).
- Isoniazid. It competes with a B6-dependent enzyme needed for the conversion and also interferes with turning niacin into NAD (ODS).
- Maize. The niacin in some grains is bound, and ODS puts its bioavailability at only about 30%. Maize is also low in tryptophan.
- Jowar (sorghum). It has enough tryptophan, but Indian textbooks (Satyanarayana; Vasudevan) teach that its excess leucine interferes with turning tryptophan into niacin.
ODS adds one more link: low riboflavin, pyridoxine or iron also slows the tryptophan-to-niacin pathway, because its enzymes depend on them.
Caption: Pellagra stems test the route to NAD: diet, tryptophan supply or the conversion step.
Clue 4: Isoniazid and Tingling Feet: What Does B6 Have to Do With It?
Pyridoxine (B6) works as pyridoxal phosphate (PLP), the coenzyme for transamination and many decarboxylations. Isoniazid binds and depletes it, which causes peripheral neuropathy. Low PLP also explains the convulsions and the sideroblastic anaemia.
The NIH ODS vitamin B6 fact sheet says PLP takes part in more than 100 enzyme reactions, mostly in protein metabolism. Three of them carry most of the exam marks, and Lippincott ties each to a sign:
- Glutamate decarboxylase makes the inhibitory transmitter GABA. Less PLP means less GABA, so infants can convulse. ODS lists irritability, unusually sharp hearing and convulsive seizures in infants.
- ALA synthase, the first step of haem synthesis, needs PLP. Less haem means a sideroblastic, microcytic anaemia. ODS lists microcytic anaemia among the deficiency signs. For how this sits beside iron deficiency and thalassaemia on a report, see the microcytic anaemia lab-report guide.
- Kynureninase in the tryptophan pathway is PLP-dependent, which is why B6 deficiency also drags niacin down.
The isoniazid link runs both ways in the exam. Isoniazid causes B6-responsive neuropathy, and it can also cause pellagra through the same pathway.
One numbers trap: ODS says plasma PLP above 30 nmol/L has been the traditional marker of adequacy. The US Food and Nutrition Board, however, used 20 nmol/L to set the RDA. Both cut-offs appear in the literature.
Clue 5: Raw Eggs Every Morning: What Does Biotin Deficiency Look Like?
Biotin (B7) is the coenzyme for carboxylases, the enzymes that add CO2. Avidin in raw egg white binds biotin and blocks its absorption. Cooking the egg denatures avidin.
The NIH ODS biotin fact sheet names the five carboxylases: propionyl-CoA carboxylase, pyruvate carboxylase, methylcrotonyl-CoA carboxylase and the two acetyl-CoA carboxylases. That gives you the one-line mechanism: no biotin, no carboxylation, so fatty acid synthesis, gluconeogenesis and some amino acid breakdown all slow.
The signs, per ODS, include thinning hair, a scaly red rash around the eyes, nose, mouth and perineum, conjunctivitis, ketolactic acidosis, seizures and brittle nails. ODS also stresses that deficiency is rare in people eating a normal mixed diet. That is why the exam's stem almost always says "raw eggs".
A newer twist: very high biotin intake can distort lab tests that use biotin-streptavidin chemistry. ODS describes falsely abnormal thyroid results. It is not yet a classic NEET PG point, but it shows up in clinical biochemistry questions.
Clue 6: Megaloblastic Anaemia: How Do You Tell B12 From Folate?
B12 and folate deficiency cause the same megaloblastic anaemia. Measure methylmalonic acid (MMA) and homocysteine: both rise in B12 deficiency, while folate deficiency raises homocysteine but typically leaves MMA normal.
The logic comes from two enzymes. The NIH ODS vitamin B12 fact sheet says B12 is a cofactor for methionine synthase and methylmalonyl-CoA mutase. Methionine synthase needs both B12 and folate (as 5-methyltetrahydrofolate) to turn homocysteine into methionine.
The mutase needs only B12. So:
| Test | B12 deficiency | Folate deficiency |
|---|---|---|
| Megaloblastic anaemia | Yes | Yes |
| Homocysteine | Raised | Raised |
| Methylmalonic acid | Raised | Typically normal |
| Spinal cord signs | Yes (subacute combined degeneration) | Not a feature |
A 2026 Australian Prescriber review by Darya Bedz and Cecily Forsyth sets out the MMA and homocysteine pattern. Bedz is a medical registrar at Wyong Hospital, New South Wales; Forsyth is a haematologist at Wyong Hospital and Central Coast Haematology. They add caveats: homocysteine also rises in B6 deficiency, chronic kidney disease and hypothyroidism, and MMA can rise in kidney disease. Their rule-out line is blunt: "Normal methylmalonic acid and homocysteine concentrations exclude vitamin B12 deficiency."
ODS calls MMA the most sensitive B12 marker. It uses MMA to confirm deficiency when serum B12 sits between 150 and 399 pg/mL, and treats values below 200 or 250 pg/mL as subnormal.
The folate trap is the reason B12 deficiency looks like folate deficiency. Harper's explains it: folate enters cells as 5-methyltetrahydrofolate, and only methionine synthase, a B12 enzyme, can release it. Without B12, folate stays "trapped" in the methyl form. DNA synthesis stalls, and the marrow turns megaloblastic.
Subacute combined degeneration is the B12-only clue. Bedz and Forsyth describe loss of vibration and proprioception, and warn that it may not fully reverse even after correction. A 2026 Advances in Nutrition expert perspective led by Jean-Louis Guéant stresses telling the two apart, so that folic acid is not given inappropriately in B12 deficiency. Harrison's explains the danger: folic acid alone can improve the blood picture while the nerve damage goes on.
Neural tube defects are the folate clue in obstetric stems. ODS notes that the US began fortifying grain products with folic acid in 1998 to reduce them. Bedz and Forsyth add that maternal B12 deficiency also raises the risk.
Caption: Two enzymes, two markers: homocysteine is shared, MMA belongs to B12.
Clue 7: Bleeding Gums and Corkscrew Hairs: Why Does Vitamin C Break Collagen?
Vitamin C keeps the iron in prolyl and lysyl hydroxylases in its active, reduced form. Without it, collagen chains are poorly hydroxylated and the triple helix is unstable. The result: bleeding gums, perifollicular haemorrhage and poor wound healing.
Harper's and Lippincott place the defect at the hydroxylation of proline and lysine. Hydroxyproline holds the triple helix together, and hydroxylysine allows the cross-links. So the weakest tissues are blood vessel walls, gums, skin and healing wounds.
The NIH ODS vitamin C fact sheet says scurvy signs can appear within 1 month of taking in less than about 10 mg/day. It lists petechiae, ecchymoses, purpura, joint pain, poor wound healing, hyperkeratosis and corkscrew hairs, and warns that untreated scurvy can be fatal.
How common is each sign? A 2022 Nutrients review by Sandra Trapani and colleagues pooled 166 children with scurvy from 15 published reports.
Mucosal signs appeared in 57%, gum bleeding in 43% and gum swelling in 27%. Petechiae or bruises appeared in 39%, but perifollicular haemorrhage in only 7% and corkscrew hairs in only 3%. These are paediatric data, but the lesson carries over: the "classic" skin signs are often absent.
Caption: In pooled paediatric cases, gums gave scurvy away far more often than corkscrew hairs.
Can You Decode These Eight Clues in Two Minutes?
Cover the answers and name the vitamin for each clue. Each miss sends you back to one clue section above.
- A man who drinks heavily, is confused and has nystagmus; the junior wants a dextrose drip first. Thiamine first (Clue 1).
- Raised erythrocyte transketolase TPP effect above 25%. Thiamine deficiency (Clue 1).
- Magenta tongue and corneal vascularisation. Riboflavin (Clue 2).
- Recurrent flushing, diarrhoea and a rash on sun-exposed skin. Carcinoid syndrome causing pellagra (Clue 3).
- A patient on anti-tubercular therapy with burning feet. Isoniazid-induced B6 deficiency (Clue 4).
- Microcytic anaemia with ringed sideroblasts in the marrow. B6, through ALA synthase (Clue 4).
- Megaloblastic anaemia, raised homocysteine, normal MMA. Folate deficiency (Clue 6).
- Swollen bleeding gums and poor wound healing in a man who eats no fruit or vegetables. Vitamin C, prolyl and lysyl hydroxylation (Clue 7).
Frequently Asked Questions
Which water-soluble vitamins matter most for NEET PG?
Thiamine, niacin, pyridoxine, B12, folate and vitamin C give examiners the most mechanisms to test, through TPP, NAD, PLP, methylcobalamin, tetrahydrofolate and ascorbate. Riboflavin and biotin tend to appear as single-clue questions.
Why is thiamine given before glucose in a malnourished or alcoholic patient?
Glucose increases the flow of pyruvate through TPP-dependent enzymes and uses up the remaining thiamine. That can worsen Wernicke encephalopathy. The EFNS advice, cited in the Li and Xing review, is not to give glucose before thiamine.
What causes pellagra apart from a low-niacin diet?
Hartnup disease, carcinoid syndrome and isoniazid are the classic non-diet causes. Each reduces the tryptophan available for niacin or blocks its conversion. In India, a jowar-based diet is the textbook dietary cause, because of its excess leucine.
How do you differentiate B12 deficiency from folate deficiency?
Both cause megaloblastic anaemia and raise homocysteine. Methylmalonic acid rises in B12 deficiency but is typically normal in folate deficiency. Spinal cord signs such as loss of vibration sense point to B12.
What is the folate trap?
Without B12, methionine synthase cannot remove the methyl group from 5-methyltetrahydrofolate. Folate gets stuck in that form, so the cell is short of usable folate even when total folate is normal. The result is megaloblastic anaemia.
The Take-Home Line
Do not memorise eight vitamins as eight lists. Memorise eight coenzymes and the enzymes they serve, and let the clue in the stem point you to the right one.
Run the chain once more tonight: TPP, FAD, NAD, PLP, biotin, tetrahydrofolate, cobalamin, ascorbate. Redo the eight-clue drill in two days. When it feels automatic, practise B-complex vitamin MCQs to check whether the clue still jumps out when it is buried in a long clinical stem.
Educational disclaimer: This content is for exam preparation and educational purposes only and is not medical advice. Always refer to the official NBEMS (natboard.edu.in), AIIMS (aiimsexams.ac.in) and NMC (nmc.org.in) notifications for the latest exam pattern, dates, eligibility and cut-offs.