Which of the following drugs does not cross placenta?

PGIMER Sangrur NO - 2023

Pharmacology

Easy

Appeared in: PGIMER Sangrur NO - 2023

Explanation

Heparin is a large polysaccharide molecule with a high molecular weight (often exceeding 1000 daltons).
Drugs with a molecular weight greater than 1000 daltons cross the placenta very poorly.
Heparin is also highly ionized (negatively charged) and water-soluble, which further limits its ability to pass through the lipid membranes of the placenta.
Due to these properties, Heparin remains in the maternal circulation and does not affect the fetus, making it the preferred anticoagulant during pregnancy.

Option A: Warfarin is a small, lipid-soluble molecule that readily crosses the placenta and can cause significant harm to the developing fetus.
Option C: Morphine is an opioid that easily crosses the placenta due to its low molecular weight and lipid solubility, posing a risk of respiratory depression to the neonate if used near delivery.
Option D: Naloxone is an opioid antagonist that crosses the placenta. Its presence in the fetal circulation is what allows it to reverse opioid-induced respiratory depression in a newborn.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Placental transfer of drugs and their effects during pregnancy as background academic context rather than a clinical decision trigger.
Understanding which drugs cross the placenta is a fundamental safety principle in nursing to prevent teratogenic effects and adverse fetal outcomes.
Nurses must be able to educate pregnant patients on why certain medications are changed during pregnancy. For example, explaining that the switch from oral Warfarin to injectable Heparin is to protect the baby.
This knowledge is crucial for managing pregnant patients with conditions requiring anticoagulation, such as deep vein thrombosis (DVT), pulmonary embolism (PE), or mechanical heart valves.

How to Approach the Question

Identify the keywords in the question: 'drug', 'does NOT', and 'cross placenta'. The negative framing ('NOT') is the most important clue.
The question requires you to recall the pharmacological principles of drug transfer across the placenta.
Think about the main factors that prevent a drug from crossing the placenta: high molecular weight, being water-soluble (not lipid-soluble), and being highly protein-bound.
Evaluate each option based on these principles. Heparin is well-known in pharmacology as a large molecule. Warfarin, Morphine, and Naloxone are smaller molecules known to have effects on the fetus, which implies they must cross the placenta.
Conclude that Heparin is the exception because its large size and polar nature prevent it from passing through the placental barrier.

Concept Tested & Keywords

Concept Tested: Placental transfer of drugs and their effects during pregnancy.
Stem keywords: drugs, cross placenta
Lead-in keywords: does not
Negative lead-in flag: The question asks to identify the drug that is an exception and does NOT cross the placental barrier.

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E6 Pharmacology Katzung 16e p. 1629-1631

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