A 68-year-old man with type 2 diabetes mellitus comes to the physician because of a 5-month history of episodic palpitations, dizziness, and fatigue. His pulse is 134/min and irregularly irregular, and his blood pressure is 165/92 mm Hg. An ECG shows a narrow complex tachycardia with absent P waves. He is prescribed a drug that decreases the long-term risk of thromboembolic complications by inhibiting the extrinsic pathway of the coagulation cascade. The expected beneficial effect of this drug is most likely due to which of the following actions?

Inhibit the reduction of vitamin K

Activate gamma-glutamyl carboxylase

Inhibit the phosphorylation of glutamate on the factor II precursor

Activate factor VII calcium-binding sites

Inhibit the absorption of vitamin K

An investigator is studying nutritional deficiencies in humans. A group of healthy volunteers are started on a diet deficient in pantothenic acid. After 4 weeks, several of the volunteers develop irritability, abdominal cramps, and burning paresthesias of their feet. These symptoms are fully reversed after reintroduction of pantothenic acid to their diet. The function of which of the following enzymes was most likely impaired in the volunteers during the study?

Alpha-ketoglutarate dehydrogenase

A 29-year-old Mediterranean man presents to the clinic for fatigue and lightheadedness for the past week. He reports an inability to exercise as his heart would beat extremely fast. He was recently diagnosed with active tuberculosis and started on treatment 2 weeks ago. He denies fever, weight loss, vision changes, chest pain, dyspnea, or bloody/dark stools. A physical examination is unremarkable. A peripheral blood smear is shown in figure A. What is the most likely explanation for this patient’s symptoms?

Drug-induced deficiency in vitamin B6

Hereditary mutations of beta-globin

Inhibition of ferrochelatase and ALA dehydratase

Abnormally low level of glutathione activity

A 58-year-old man presents to the Emergency Department after 3 hours of intense suprapubic pain associated with inability to urinate for the past day or two. His medical history is relevant for benign prostatic hyperplasia (BPH) that has been under treatment with prazosin and tadalafil. Upon admission, he is found to have a blood pressure of 180/100 mm Hg, a pulse of 80/min, a respiratory rate of 23/min, and a temperature of 36.5°C (97.7°F). He weighs 84 kg (185.1 lb) and is 175 cm (5 ft 7 in) tall. Physical exam, he has suprapubic tenderness. A bladder scan reveals 700 ml of urine. A Foley catheter is inserted and the urine is drained. Initial laboratory tests and their follow up 8 hours after admission are shown below. Admission 8 hours after admission Serum potassium 4.2 mmol/L Serum potassium 4.0 mmol/L Serum sodium 140 mmol/L Serum sodium 142 mmol/L Serum chloride 102 mmol/L Serum chloride 110 mmol/L Serum creatinine 1.4 mg/dL Serum creatinine 1.6 mg/dL Serum blood urea nitrogen 64 mg/dL Serum blood urea nitrogen 62 mg/dL Urine output 250 mL Urine output 260 mL A senior attending suggests a consultation with Nephrology. Which of the following best justifies this suggestion?

Estimated glomerular filtration rate (eGFR)

Serum blood urea nitrogen (BUN)

A previously healthy 18-year-old woman comes to the physician because of a 2-day history of swelling and itchiness of her mouth and lips. It decreases when she eats cold foods such as frozen fruit. Four days ago, she underwent orthodontic wire-placement on her upper and lower teeth. Since then, she has been taking ibuprofen twice daily for the pain. For the past 6 months, she has been on a strict vegan diet. She is sexually active with one partner and uses condoms consistently. She had chickenpox that resolved spontaneously when she was 6 years old. Her vitals are within normal limits. Examination shows diffuse erythema and edema of the buccal mucosa with multiple serous vesicles and shallow ulcers. Stroking the skin with pressure does not cause blistering of the skin. The remainder of the examination shows no abnormalities. Which of the following is the most likely cause of these symptoms?

Reactivation of varicella zoster virus

An 8-year-old boy presents with recurrent infections including multiple episodes of pneumonia and diarrhea. He reports difficulty seeing in the dark. Physical examination reveals white patches on the sclera and conjunctival dryness. What is the most likely cause of these findings?

Spinocerebellar ataxia (SCA) type 1

Vitamin cofactor functions in metabolism for USMLE Step 3: High-Yield Biochemistry Lessons

B Vitamins (1, 2, 3) - The Energy Crew

Citric Acid Cycle & Cofactors

B1 (Thiamine → TPP)
- Cofactor for dehydrogenase enzymes.
- Key reactions: Pyruvate DH, α-ketoglutarate DH, Transketolase.
- 📌 ATP Mnemonic: Alpha-ketoglutarate, Transketolase, Pyruvate DH.
B2 (Riboflavin → FAD, FMN)
- Redox reactions (succinate dehydrogenase).
- Essential for the electron transport chain (Complex I & II).
B3 (Niacin → NAD+, NADP+)
- Redox reactions (dehydrogenases).
- Synthesized from Tryptophan; requires B6.

⭐ Niacin can be used to treat hyperlipidemia as it decreases hepatic VLDL synthesis.

B Vitamins (5, 6, 7) - The Biosynthesis Brigade

B5 (Pantothenic Acid): Component of Coenzyme A (CoA) and fatty acid synthase. Essential for acyl group transfers.
B6 (Pyridoxine → PLP): Critical for transamination (ALT/AST), decarboxylation, and heme synthesis. Deficiency can be induced by isoniazid.
B7 (Biotin): Cofactor for carboxylation enzymes (e.g., pyruvate carboxylase in gluconeogenesis). 📌 Mnemonic: "Buy-a-tin of CO₂" for carboxylation.

⭐ Isoniazid, a primary TB medication, can cause a functional Vitamin B6 deficiency by forming an inactive derivative with PLP, leading to peripheral neuropathy and sideroblastic anemia.

B Vitamins (9, 12) - The Methyl Movers

Folate (B9) & Cobalamin (B12) are crucial for one-carbon metabolism (methylation), primarily for synthesizing DNA and amino acids.
Folate (B9) / Tetrahydrofolate (THF):
- Function: Transfers 1-carbon units for purine and thymidine synthesis.
- Deficiency: Causes megaloblastic anemia, ↑ homocysteine, and potential neural tube defects. Methylmalonic acid (MMA) is normal.
Cobalamin (B12):
- Function: Cofactor for methionine synthase (regenerates THF) and methylmalonyl-CoA mutase.
- Deficiency: Causes megaloblastic anemia plus neurological deficits. Traps folate in an unusable form (folate trap).

⭐ Differentiating deficiency: ↑ MMA is specific to B12 deficiency.

Folate, B12, B6 metabolism pathways and homocysteine

Vitamin K & Minerals - The Supporting Cast

Vitamin K (Phytonadione, Phylloquinone)
- Activates clotting factors by $\gamma$-carboxylating glutamic acid residues.
- Factors: II, VII, IX, X, and proteins C & S.
- Deficiency: ↑ PT & aPTT, normal bleeding time. Hemorrhagic disease of the newborn.
Zinc
- Cofactor for 100+ enzymes (e.g., carbonic anhydrase, collagenase).
- Deficiency: Acrodermatitis enteropathica, poor wound healing, dysgeusia, anosmia.
Copper
- Cofactor for lysyl oxidase (collagen cross-linking), tyrosinase (melanin), cytochrome c oxidase.
- Deficiency: Microcytic anemia, bone demineralization.
Selenium
- Cofactor for glutathione peroxidase (antioxidant).
- Deficiency: Keshan disease (cardiomyopathy).

⭐ Warfarin Antagonism: Warfarin inhibits vitamin K epoxide reductase, blocking the regeneration of active vitamin K, thus preventing the synthesis of functional clotting factors.

Vitamin K functions in human body systems

High‑Yield Points - ⚡ Biggest Takeaways

B1 (Thiamine) is a critical cofactor for dehydrogenase enzymes (e.g., pyruvate dehydrogenase).

B2 (Riboflavin) and B3 (Niacin) form FAD and NAD, essential for all redox reactions.

B5 (Pantothenate) is the core component of Coenzyme A (CoA) and fatty acid synthase.

B6 (Pyridoxine) is vital for transamination and decarboxylation in amino acid metabolism.

B7 (Biotin) acts as a CO2 carrier in carboxylation reactions.

B9 (Folate) and B12 (Cobalamin) are key for one-carbon transfers, crucial for nucleotide synthesis.

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