Pharmacokinetics and Pharmacodynamics Practice Questions

Q: Which NSAID undergoes enterohepatic circulation?

Piroxicam. **Explanation:** **Piroxicam** is a long-acting oxicam derivative. The primary reason for its prolonged half-life (approximately 50 hours) is its extensive **enterohepatic circulation**. After being absorbed and metabolized in the liver, a significant portion of the drug is excreted into the bile and subsequently reabsorbed from the gastrointestinal tract back into the systemic circulation. This recycling mechanism allows the drug to maintain therapeutic plasma concentrations for a longer duration, enabling convenient **once-daily dosing**. **Analysis of Incorrect Options:** * **Phenylbutazone:** While it has a long half-life (50–100 hours), this is primarily due to its slow metabolism and high plasma protein binding, rather than significant enterohepatic recycling. * **Aspirin:** It is a short-acting NSAID that is rapidly hydrolyzed to salicylic acid. It follows first-order kinetics at low doses and zero-order kinetics at high doses, but does not undergo enterohepatic circulation. * **Ibuprofen:** A propionic acid derivative with a short half-life (approx. 2 hours). It is rapidly metabolized and excreted in the urine, requiring multiple daily doses. **High-Yield Clinical Pearls for NEET-PG:** * **Indomethacin** is another classic example of an NSAID that undergoes significant enterohepatic circulation (often associated with GI side effects). * **Piroxicam** is associated with a higher risk of gastric ulcers and bleeding compared to other NSAIDs due to its long duration of action. * **Clinical Tip:** Drugs undergoing enterohepatic circulation often show a "second peak" in plasma concentration curves and are more susceptible to interactions with broad-spectrum antibiotics (which disrupt gut flora required for deconjugation).

Q: Which of the following drugs is not administered by the intradermal route?

Insulin. **Explanation:** The **intradermal (ID) route** involves injecting a small volume of medication into the dermis, just below the epidermis. This route is primarily used for diagnostic purposes, sensitivity testing, and specific vaccinations because the dermis has a limited blood supply, leading to slow systemic absorption. **Why Insulin is the Correct Answer:** Insulin is traditionally administered via the **subcutaneous (SC) route**. The subcutaneous tissue (fatty layer) allows for a consistent, slow, and predictable absorption rate into the systemic circulation. Injecting insulin intradermally would result in erratic absorption and significant pain, making it clinically inappropriate for routine glucose management. **Analysis of Incorrect Options:** * **BCG (Bacillus Calmette-Guérin):** This is the classic example of an ID injection. It must be given intradermally to induce a local delayed-type hypersensitivity reaction; accidental subcutaneous injection can lead to abscess formation. * **Mantoux Test:** Used for tuberculosis screening (Tuberculin Skin Test), PPD is injected intradermally to observe for induration, which indicates a T-cell mediated immune response. * **Drug Sensitivity Injection:** Skin prick or ID tests (e.g., for Penicillin) are used to detect immediate IgE-mediated hypersensitivity (Type I) before administering a full dose. **High-Yield Clinical Pearls for NEET-PG:** * **Angle of Injection:** ID injections are given at a **10–15 degree angle**, while SC injections are typically given at **45–90 degrees**. * **Volume:** ID route accommodates very small volumes (usually **0.1 ml**). * **Rabies Vaccine:** The **Thai Red Cross regimen** (ID) is a cost-effective alternative to the intramuscular route, utilizing the high density of antigen-presenting cells (Langerhans cells) in the skin. * **Other ID Vaccines:** Smallpox vaccine (multiple puncture) and some newer Influenza vaccines.

Q: Epinephrine acts by stimulating which of the following?

Adenyl cyclase. **Explanation:** Epinephrine (Adrenaline) is a potent catecholamine that acts as a non-selective agonist at alpha ($\alpha$) and beta ($\beta$) adrenergic receptors. The correct answer is **Adenyl cyclase** because the primary mechanism for epinephrine’s most significant physiological effects (especially via $\beta$-receptors) involves the **G-protein coupled receptor (GPCR)** pathway. 1. **Why Adenyl Cyclase is correct:** When epinephrine binds to $\beta_1, \beta_2,$ or $\beta_3$ receptors, it activates a stimulatory G-protein ($G_s$). This $G_s$ protein activates the enzyme **Adenyl cyclase**, which converts ATP into **cyclic AMP (cAMP)**. cAMP then acts as a second messenger to activate Protein Kinase A (PKA), leading to effects like increased heart rate (positive inotropy/chronotropy) and bronchodilation. 2. **Why Phosphodiesterase is incorrect:** Phosphodiesterase (PDE) is the enzyme responsible for the *breakdown* of cAMP into 5'-AMP. Epinephrine increases cAMP levels; it does not stimulate the enzyme that destroys it. (Note: PDE inhibitors like Theophylline or Milrinone are used to keep cAMP levels high). 3. **Why Phospholipase is incorrect:** Phospholipase C is typically associated with $G_q$-coupled receptors (like $\alpha_1$). While epinephrine does act on $\alpha_1$ receptors, its classic systemic "fight or flight" profile is dominated by the Adenyl cyclase-cAMP pathway. **High-Yield Clinical Pearls for NEET-PG:** * **Receptor Potency:** At low doses, epinephrine has a higher affinity for $\beta_2$ receptors (vasodilation); at high doses, $\alpha_1$ effects predominate (vasoconstriction). * **Drug of Choice:** Epinephrine (1:1000 IM) is the drug of choice for **Anaphylactic Shock**. * **Glucagon connection:** Like epinephrine, Glucagon also acts via Adenyl cyclase, which is why it is used as an antidote for Beta-blocker poisoning.

Q: Which of the following is a pro-drug?

Cyclophosphamide. **Explanation:** **Cyclophosphamide** is the correct answer because it is a classic example of a **pro-drug**. A pro-drug is a pharmacologically inactive compound that must undergo metabolic conversion (biotransformation) within the body to become an active metabolite. Cyclophosphamide, an alkylating agent used in chemotherapy, is inactive in its parent form. It requires activation by **hepatic cytochrome P450 enzymes** (specifically CYP2B6) to form 4-hydroxycyclophosphamide and aldophosphamide, which eventually break down into the active cytotoxic moiety, **phosphoramide mustard**. **Analysis of Incorrect Options:** * **Lisinopril:** This is a high-yield exception among ACE inhibitors. While most ACE inhibitors (like Enalapril or Ramipril) are pro-drugs, **Lisinopril and Captopril** are active in their parent form and do not require hepatic activation. * **Metoclopramide:** This is a prokinetic and antiemetic agent that is active upon administration. It acts directly as a $D_2$ receptor antagonist. * **Ranitidine:** This is an $H_2$ receptor antagonist used to reduce gastric acid secretion. It is an active drug and does not require metabolic activation to exert its effect. **NEET-PG Clinical Pearls:** * **Acrolein:** The activation of Cyclophosphamide also produces acrolein, a toxic metabolite responsible for **hemorrhagic cystitis**. This is prevented by administering **MESNA** (2-Mercaptoethane sulfonate) and aggressive hydration. * **Pro-drug Exceptions:** Remember that most ACE inhibitors are pro-drugs *except* Lisinopril and Captopril. * **Common Pro-drugs to remember:** Levodopa (to Dopamine), Enalapril (to Enalaprilat), Clopidogrel, Terfenadine (to Fexofenadine), and Prednisone (to Prednisolone).

Q: Which of the following factors has the maximum effect on the filtration of a drug by the glomerulus?

Plasma protein binding. **Explanation:** **Glomerular filtration** is a passive process where drugs are filtered through the capillary pores of the glomerulus [1]. The primary determinant of this process is the **molecular size** and the **free (unbound) fraction** of the drug [2]. 1. **Why Plasma Protein Binding is Correct:** Drugs in the plasma exist in two forms: bound to proteins (like albumin) and free. The glomerular filtration barrier acts as a sieve that prevents large molecules like plasma proteins from passing through [3]. Consequently, any drug molecule bound to a protein becomes too large to be filtered. Therefore, the **rate of filtration is directly proportional to the free fraction** of the drug [2]. High protein binding significantly limits glomerular filtration. 2. **Why Other Options are Incorrect:** * **Lipid Solubility & Degree of Ionization:** Unlike tubular reabsorption (which is highly dependent on lipid solubility and pH-dependent ionization), glomerular filtration is a bulk flow process through large pores [1]. It does not require the drug to cross lipid membranes; thus, lipid solubility and ionization have **no significant effect** on filtration. * **Rate of Tubular Secretion:** This is a separate process occurring in the renal tubules via active transporters (e.g., OAT, OCT). While it contributes to the total renal clearance, it does not influence the filtration occurring at the glomerulus [2]. **High-Yield Clinical Pearls for NEET-PG:** * **Glomerular Filtration Rate (GFR):** Only the **unbound** drug is filtered [2]. * **Tubular Secretion:** Can clear both **bound and unbound** drugs (e.g., Penicillin is highly protein-bound but rapidly cleared via secretion) [2]. * **Tubular Reabsorption:** Highly dependent on **Lipid solubility** and **pH** (Ion trapping). * **Formula:** Renal Clearance = (Filtration + Secretion) – Reabsorption.

Q: Which of the following drugs is metabolized through glycin(e) conjugation?

Nicotinic acid. **Explanation:** **Correct Option: D (Nicotinic acid)** Metabolism occurs via two main phases: Phase I (Functionalization) and Phase II (Conjugation). **Glycine conjugation** is a specific Phase II pathway used for compounds containing a carboxylic acid group. **Nicotinic acid (Niacin)** and **Salicylates** (Aspirin) are the classic examples of drugs metabolized through this pathway. In this process, the drug is activated to a Coenzyme A thioester, which then reacts with glycine to form a water-soluble conjugate (e.g., nicotinuric acid) for renal excretion. **Analysis of Incorrect Options:** * **A. Benzodiazepines:** These drugs primarily undergo Phase I oxidation (via CYP3A4/2C19) followed by **Glucuronidation** (Phase II). Note: "LOT" drugs (Lorazepam, Oxazepam, Temazepam) undergo direct glucuronidation, bypassing Phase I. * **B. Atazanavir:** This protease inhibitor is extensively metabolized in the liver primarily by the **CYP3A4** isoenzyme (Phase I). * **C. Ifosfamide:** This alkylating agent is a prodrug that requires activation by **CYP3A4 and CYP2B6** (Phase I) to form its active metabolite, ifosfamide mustard. **High-Yield Clinical Pearls for NEET-PG:** * **Phase II Conjugation Mnemonic:** Remember **"S-G-A-G"** (Sulfation, Glucuronidation, Acetylation, Glycine conjugation). * **Glucuronidation** is the most common Phase II reaction. * **Acetylation** shows genetic polymorphism (Fast vs. Slow acetylators); drugs involved include **S**ulfonamides, **H**ydralazine, **I**soniazid, and **P**rocainamide (**SHIP**). * **Gray Baby Syndrome** occurs in neonates due to a deficiency of **UDP-glucuronyltransferase**, preventing the conjugation of Chloramphenicol.

Question 1

Which NSAID undergoes enterohepatic circulation?

Accepted Answer

Piroxicam

Answer

Phenylbutazone

Answer

Aspirin

Answer

Ibuprofen

Question 2

What is the bioavailability?

Accepted Answer

(AUC of oral administration / AUC of intravenous administration) * 100

Answer

Area under the plasma concentration-time curve (AUC) of oral administration / AUC of intravenous administration

Answer

AUC of intravenous administration / AUC of oral administration

Answer

(AUC of intravenous administration / AUC of oral administration) * 100

Question 3

Which of the following drugs is not administered by the intradermal route?

Accepted Answer

Insulin

Answer

BCG

Answer

Mantoux

Answer

Drug sensitivity injection

Question 4

Epinephrine acts by stimulating which of the following?

Accepted Answer

Adenyl cyclase

Answer

Phosphodiesterase

Answer

Phospholipase

Answer

None of the above

Question 5

Which of the following is NOT a characteristic of active drug transport?

Accepted Answer

Carrier transport is nonsaturable

Answer

Energy dependent

Answer

Makes use of specific transporter proteins

Answer

Carries the drug against the concentration gradient

Question 6

In which body compartment is a plasma protein-bound drug primarily distributed?

Accepted Answer

Intravascular fluid

Answer

Extracellular fluid

Answer

Interstitial fluid

Answer

Extravascular fluid

Question 7

Which of the following is a pro-drug?

Accepted Answer

Cyclophosphamide

Answer

Lisinopril

Answer

Metochlopramide

Answer

Ranitidine

Question 8

What is a prodrug?

Accepted Answer

An inactive drug that is transformed in the body to an active metabolite.

Answer

The prototype member of a class of drugs.

Answer

The oldest member of a class of drugs.

Answer

A drug that is stored in body tissues and is then gradually released.

Question 9

Which of the following factors has the maximum effect on the filtration of a drug by the glomerulus?

Accepted Answer

Plasma protein binding

Answer

Lipid solubility

Answer

Degree of ionization

Answer

Rate of tubular secretion

Question 10

Which of the following drugs is metabolized through glycin(e) conjugation?

Accepted Answer

Nicotinic acid

Answer

Benzodiazepines

Answer

Atzanavir

Answer

Ifosfamide

Pharmacokinetics and Pharmacodynamics — MCQs

Pharmacokinetics and Pharmacodynamics — MCQs

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